A simple screening method for the detection of lymphocyte cytotoxic antisera is described utilising lymphocytes from routine blood donor samples and sera from samples submitted for prenatal screening. The scheme can be run by one experienced worker. Fourteen thousand sera were screened in 18 months; 911 subjects were found to have cytotoxic antibodies with a 70% minimum kill and from these women 164 donations of cytotoxic antisera were obtained (an average of 150 ml/donation). Sixteen of these antisera were found monospecific by us and confirmed by the National Tissue Typing Research Laboratory. A further 16 were found useful in tissue typing. The various specificities of 464 random cytotoxic positive sera were evaluated and the incidence of cytotoxic activity, through various numbers of pregnancies to five full term deliveries, is recorded. The highest incidence is found at the 3rd, 4th, and 5th full term deliveries. An initial scheme for a tissue-typed blood donor panel is included.