Sera from about 1000 cows were tested for cytotoxicity against a panel of up to 100 lymphocyte samples. Cytotoxic antibodies presumably resulting from transplacental immunization of the cow by her calf were found in about 45% of these sera. The antibody titers of sera from parous cows rarely exceed 4(2), some persisted for over one year, but decreased notably at calving. Thirty-five immune sera were also produced by alloimmunization with lymphocytes. They usually reached peak titers of up to 4(4) at 2 or 3 weeks after the initial immunization. Subsequent immunizations produced sera with very high titers but they were much more polyspecific. High-titered antibodies were also produced by skin graft recipients. Useful cytotoxic antibodies were found in 19 of 111 colostrum whey samples. Studies on 13 dam-calf pairs showed that the newborn calf may acquire cytotoxic antibodies from its mother's colostrum, but the only cytotoxic antibodies detectable in this calf's serum are those not directed against its own lymphocyte antigens. It is concluded that efficient lymphocyte typing requires antibodies from a variety of sources.