Rat syngeneic and allogeneic chondrocytes were transplanted intramuscularly or into defects prepared in articular cartilage (intracartilaginous transplants). Recipients of allogeneic transplants received cyclosporin A (CsA), cladribine (2-chlorodeoxyadenosine, 2-CdA), or both drugs in combination. Transplants were taken for examination after 5 weeks. Cartilage formed intramuscularly by syngeneic chondrocytes was ossified. Allogeneic cartilage was resorbed by infiltrating cells. CsA or 2-CdA partially suppressed, and both these agents in combination strongly suppressed, formation of infiltrations. Both syngeneic and allogeneic chondrocytes formed cartilage in joint surface defects but only allogeneic cartilage was attacked by infiltrating cells. CsA + 2-CdA treatment slightly decreased intensity of infiltrations but did not prevent cartilage resorption. Antichondrocyte response was studied by evaluation of spleen mononuclear cells (SMC) stimulation in mixed splenocyte-chondrocyte cultures and by detection of antichondrocyte cytotoxic antibodies. SMC stimulation index (SI) was calculated separately for syngeneic and allogeneic chondrocytes. Comparison of SMC SI for syngeneic and allogeneic chondrocytes indicated lack of stimulation of SMC from control or syngeneic transplant recipients and significant stimulation of SMC from recipients of allogeneic transplants. SMC from animals treated with CsA + 2-CdA were not stimulated. Additional experiments aiming at an explanation of the lack of stimulation of SMC from intact animals by syngeneic chondrocytes reported in this work and contrary to other findings disclosed that it was caused by the use of collagenase solution containing N alpha-p-tosyl-l-lysine chloromethyl ketone for chondrocyte isolation. Spontaneous antichondrocyte cytotoxic antibody activity was found in intact rats raised only in sera from recipients of allogeneic intramuscular transplants without immunosuppression. Thus, strong immunosuppressive treatment of rats with allogeneic chondrocyte transplants was more effective in relation to the general immunological response than to the local reaction.