Sera of 134 recipients awaiting 1st renal transplants were tested for cytotoxic antibody by 3 methods to assess the different techniques and various factors in the techniques. The use of only 1 technique would have failed to detect some of the antibodies found in the sera of 35 patients; it was necessary to use a 37.degree. C test and the NIH [National Institute of Health] 20.degree. C technique to demonstrate them. Results on 402 serial samples from 108 patients indicate the importance, because of fluctuation in antibody levels, of using more than 1 sample, i.e., previous sample in addition to a fresh specimen from any patient when crossmatching prior to renal transplantation. Lymphocyte suspensions from 44 of 118 cadaver donors were unsatisfactory for accurate cytotoxic crossmatching due to a high background of dead cells. DNase renders such suspensions suitable for use and should be used in such cases. If more than 1 technique were generally used, improved detection of cytotoxic activity would probably result and would help to resolve present conflicting opinions about the effects of cytotoxic antibody on graft survival. Results are given of 19 patients who had formed cytotoxic antibody prior to renal transplantation. Of these, 18 have surviving grafts.