SYNOPSIS Five hundred and eighteen human sera containing known red cell antibodies were tested for lymphocytotoxic antibodies and 81 sera were found to contain them. Thirty-nine antibodies were fully characterized. The frequencies of anti-I, K, Vw, and Wra were significantly greater in those of the 518 sera which also contained white cell antibodies.Four hundred and ninety-four of the 518 sera containing red cell antibodies contained anti-Rh (Boyum, 1968), the concentration being adjusted to 1 x 106 cells/ml.