The postoperative prophylactic cytolytic therapy with rabbit-ATG and OKT3 after heart transplantation are compared. The first 20 recipients were treated with ATG (5 days), the next 20 with OKT3 (14 days). The medium histological rejection grade (Texas classification) was significantly higher after 2 weeks in ATG group and after 4 weeks in OKT3 group. The linearised rejection rate was 7.1 (ATG) vs. 0.7 (OKT3) (p less than 0.005) at 2 weeks, resp. 3.6 vs. 8.6 (p less than 0.05) and 1.3 vs. 2.9 (p less than 0.005) at 4 weeks and 2 months. After 3 months there was no difference in histological rejection grade (3.6 +/- 1.5 vs. 3.3 +/- 1.7) and linearised rejection rate (2.4 vs. 2.6 per 100 days and patient) between the two groups. Severe rejections are more frequent after OKT3 (6 vs. 11) and probability of rejection free survival is higher after ATG (25% vs. 0%, resp. 21% vs. 0% after 2 resp. 3 months; p less than 0.05). In this study we find no short and medium term benefit of a rejection prophylaxis with OKT3 (14 days) compared with ATG (5 days). Acute allograft rejections in OKT3 group tend to be more severe with a higher mortality.