1. Nineteen rabbit does were inoculated each with the whole blood of a buck to which they were later mated.

2. They were observed over a period of 10 months which included three pregnancies: (a) for the appearance and behaviour of anti-red cell isoanti-body in their sera; and (6) for the effect of such an antibody on the offspring, especially those inheriting a paternal red cell antigen incompatible with their mother's antiserum.

3. The does' antisera were examined for antibodies capable of directly agglutinating red cells and for ‘incomplete’ antibodies which might sensitize cells without causing agglutination. The latter were identified by an indirect antiglobulin sensitization test.

4. Nine does formed antibodies in their sera, six of these antisera directly agglutinated the appropriate red cells, but in three cases the antibodies were demonstrated only by the indirect antiglobulin sensitization test. Of the six antisera which directly agglutinated red cells three had antibodies which caused direct agglutination of red cells undiluted or in their lower dilutions, but in higher dilutions were shown to be capable of sensitizing cells without agglutinating them.

5. Four does, who had produced antibody in their sera, had between them seven litters in which the red cells of the young inheriting a paternal antigen incompatible with their mother's antiserum reacted to a direct antiglobulin sensitization test. This reaction is discussed and it is concluded that a positive direct antiglobulin sensitization test means that maternal isoantibody is attached to the infants' red cells. The offspring whose red cells reacted positively were, nevertheless, clinically normal.

6. A proportion of the young were killed at 3 days and 24 days old. A full peripheral blood examination was made and imprints taken from the liver and spleen. The bone marrow was also examined. Specimens were taken from liver, spleen, suprarenal, kidneys, femur, and, in some cases, brain for histological examination after an autopsy had been performed. Fifteen normal rabbits were similarly examined at 3 days old and four at 24 days old.

7. The range and average figures for the various haematological examinations made are given and the findings in the experimental animals compared with the normals.

8. A few differences were found between the experimental and the normal animals as a result of this experiment. The significance of these is discussed.