Preexisting humoral immunity to donor alloantigens was investigated in 67 potential renal transplant patients on chronic hemodialysis on a continuing basis over 28 months by 2,740 cytotoxicity tests. The cytotoxicity tests were enhanced by the use of six concentrations of recipient sera, and other techniques for better assessment of immunity. The six-well crossmatch concentration profile demonstrated 596 discordant results (21.8%) from those obtained by a single-well crossmatch. Among these were 196 positive profiles which were negative in the customary 1λ well. The profile method further demonstrated a low number of false positives (32) and resolved 100 of 116 doubtful reactions. Dilutions appeared more valuable than concentrations in determining the final crossmatch. No cases of hyperacute or accelerated rejection occurred in 20 cadaver and 9 related kidney transplants.