Active enhancement was induced in inbred rats with cardiac allografts using semisoluble antigens. The optimal time of antigen pretreatment and optimal dose of semisoluble antigens were examined. The presence of serum blocking factors in the sera of rats having had allografts for a long time was examined with a macrophage migration inhibition test and lymphocyte microcytotoxicity assay. Since the blocking factors of macrophage migration inhibition were increasing on the 7th day, that day was determined to be the optimal time of antigen pretreatment. The mean survival time (MST) of cardiac allografts in untreated rats was 17.2 +/- 7.5 days. Semisoluble antigens were administered at 2 mg/kg body weight 7 days before the graft, 4 mg/kg 7 days before the graft and 2 mg/kg divided over three days, 15, 8 and 1 day before the graft, and the MSTs of cardiac allografts of rats receiving these treatments were 71.2 +/- 39.9, 62.6 +/- 42.2 and 79.3 +/- 31.0 days, respectively. The MST in each group of the treated rats was significantly longer than that of the control group (p less than 0.01). Rejection of the allograft, however, was accelerated in a group treated with 8 mg/kg 7 days before the graft (MST: 8.4 +/- 3.2 days). Serum blocking factors were detected in the sera of approximately half of the rats having cardiac allografts which survived a long time.</p
