Sera of 134 recipients awaiting first renal transplants have been tested for cytotoxic antibody by three methods in order to assess the different techniques, and various factors in the techniques have been investigated. The use of only one technique would have failed to detect some of the antibodies found in the sera of 35 patients and it was necessary to use both a 37 C test and the NIH 20 C technique in order to demonstrate them. Results on 402 serial samples from 108 patients indicate the importance, because of fluctuation in antibody levels. of using more than one 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 found to be 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. It is felt that if more than one technique were generally used, improved detection of cytotoxic activity would 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 in the Unit. Eighteen of these have surviving grafts.