An antiserum was raised in a rabbit against 0 panel red cells of mackerel. The erythrocytes of oil sardine and mackerel were tested against human blood typing sera anti A and B and also the test serum of rabbit which revealed the presence of antigens A and B. In addition, an antigen common to both the fishes and human A, B and 0 panel red cells was noted but not identifiable. The blood group B did not manifest itself clearly either in oil sardine or mackerel. The blood groups A, AB and 0 indicated the existence of genetically different groups of oil sardine and mackerel. Isoagglutinin tests revealed the presence of a reciprocal relationship with antigens A and B in both these fishes.