The present study was undertaken to acquire a rationale for clinical dose adjustment of anti-thymocyte globulin (ATG) to improve cost effectiveness and safety of graft-versus-host disease prophylaxis. The concentration of rabbit ATG in the serum of 12 patients was measured by ELISA and by the inhibitory effect on phytohaemagglutinin-induced blastogenesis. At 10 mg/ml ATG, 3H-thymidine incorporation was effectively blocked. Serial two-fold dilution of ATG showed that this effect decreased in a concentration-dependent manner and was lost at 10 ng/ml ATG. One hundred microlitres serum taken at day -1 to +22 post transplant effected significant inhibition of the phytohaemagglutinin-response with 49+/-12% c.p.m. (x +/- s.d.) on day +1 post transplant compared to 93+/-13% c.p.m. on day -1 (P
