Assays for cellular (cell-mediated lympholysis, CML) and humoral (antibody-dependent lymphocytotoxicity and complement-dependent lympholysis) evidence of graft-specific presensitization are reported in a series of 17 patients. In four of these patients, serial post-transplant assays have been correlated in detail with the clinical course. This revealed evidence of a complicated and fluctuating interplay between the cellular and humoral immune responses. These studies have not defined a pattern permitting a confident prediction of the clinical outcome in an individual patient. CML may be negative despite unequivocal serological evidence of preexisting graft-specific sensitization. A positive CML may be associated with a satisfactory clinical outcome. An abrupt but transient anamnestic increase in graft-specific antibodies following transplantation was associated with a very satisfactory clinical outcome, and suggests but does not formally prove the occurrence of an enhancement phenomenon.