1. We have discovered that Trypanblau and Trypanrot are highly efficient remedies in the treatment of canine piroplasmosis, no drug or mode of treatment having hitherto been found to exert any appreciable effect upon this very fatal disease.

2. The drugs exert a direct and observable effect upon the parasites (a) by causing the pyriform parasites to quickly disappear, and (b) in most cases, by causing the total disappearance of the parasites from microscopic observation in the peripheral blood.

3. The disappearance of the parasites from the blood is usually temporary, the parasites reappearing in small numbers after an interval of 9 to 12 days, but the dogs, as a rule, show no symptoms and gradually progress toward recovery. In our experience there is but slight loss of weight in treated animals, this being in marked contrast to what is usually observed in dogs which recover naturally. In the two dogs which died of a relapse the parasites reappeared after an interval of 4 to 5 days.

4. The experiments were conducted upon 21 dogs of all ages and of a variety of breeds. Many of the dogs were highly susceptible puppies. Twelve dogs were subjected to curative treatment, one dog received preventive treatment, and eight dogs served as controls.

5. Twelve dogs received treatment as follows:

(a) Two dogs were treated with Trypanrot and survived 111 days and 20 days respectively. In the latter case the dog was treated in an advanced stage of the disease, the parasites disappeared, and death appeared to be due to the after-effects of the malady.

Two control dogs died respectively on the 9th and 7th day after they were inoculated.

(b) Ten dogs were treated with Trypanblau and seven made a good recovery. Three of these dogs are still alive, having been respectively under observation for periods of 90, 83 and 65 days. Four of the recovered dogs died respectively of distemper on the 42nd and 43rd day, of severe mange on the 52nd day and of distemper and mange on the 69th day. We failed to cure the disease in 3 out of 10 cases treated: two small wormy ill-nourished puppies died from a relapse (on the 13th and 15th day) after they appeared to be on the road to recovery; one dog, treated only 3 hours prior to death, and when 50% of its corpuscles were infected, succumbed to the disease, but even in this case a visible effect was exerted upon the parasites by the drug.

Of the 7 control dogs, 6 died of piroplasmosis within 7–13 days after inoculation with virulent blood, and the remaining dog died of piroplasmosis on the 36th day after inoculation.

6. After the parasites disappear under the influence of the drug, the temperature usually falls to normal, but slight rises of temperature may at times occur, together with the subsequent reappearance of the parasites.

7. When the parasites reappear after treatment they occur in small numbers and can usually be detected only at the edge of the film. In such cases the parasites tend to occur in higher multiples [(PPPP) or more] within the individual corpuscles. The same appearance has been observed in one of the control dogs, which died of chronic piroplasmosis on the 36th day.

8. In the two puppies 10 and 11, in which death followed upon a relapse, the parasites reappeared after a shorter interval (4 to 5 days) than in any of the other dogs. The blood counts made during the fatal relapse are remarkable as showing a high percentage of dividing forms (D); that rapid invasion of fresh corpuscles was proceeding is also shown by the high percentage of single pyriform parasites (P) which occurred in the corpuscles.

9. One experiment is reported (Dog 13) in which Trypanblau was injected 24 hours after the animal was inoculated, with the result that no parasites appeared in the dog's blood up to the 65th day, and the dog remained perfectly well. The control dog died of piroplasmosis on the 7th day. (This experiment has since been successfully repeated.)

Further experiments of this character will be shortly reported upon.

10. Arsacetin and Soamin exert no curative effect upon canine piroplasmosis.