Total and differential cell counts were made on a total of 26272 quarter samples of foremilk taken at fortnightly intervals from cows in the Institute herd during the 12 years 1953–1965, and the samples were also examined bacteriologically.

The average cell count in each lactation up to the 7th was calculated.

The average total cell count of samples from which no staphylococci, streptococci or coliform organisms were isolated increased from the 1st to the 7th lactations from 0·19 to 0·60 million cells/ml, and from the 2nd lactation onwards the increase was due entirely to an increase in the number of polymorphs. With infected samples, however, changes in the average total cell count were caused by changes in the numbers both of polymorphs and of cells other than polymorphs.

The average total cell count of samples from which coagulase-negative staphylococci were isolated varied between 0·58 and 1·00 million/ml throughout the 7 lactations. For samples from which coagulase-positive staphylococci were isolated the cell count varied between 0·72 and 4·94 million/ml, the highest average total cell count occurring in the 5th lactation.

The average total cell count of samples from which streptococci were isolated varied between 1·48 and 4·62 million cells/ml, the highest value occurring in the 2nd lactation.