We investigated the effects of nonspecific binding on thyrotropin values obtained by radioimmunoassay in which polyethylene glycol is used as precipitant. Differences in nonspecific binding among individual samples were significant (F-test, p less than 0.001, range 5.5 to 14.1%). Non-specific binding and total serum protein were directly correlated (r = 0.472, n = 59; p less than 0.001). Nonspecific binding increased with increasing concentrations of globulins but showed no relation to albumin concentration. If globulin concentration was less than 15 g/L, precipitation of the antigen--antibody complex by polyethylene glycol was incomplete. The mean value for thyrotropin in sera from 67 healthy subjects was 2.7 (SD 0.3) milli-international units per liter (milli-int. unit/L) without individual serum nonspecific binding correction, significantly (p less than 0.005) higher than that with nonspecific binding correction (1.6, SD 0.1, milli-int. unit/L). Evidently, inter-sample variations in nonspecific binding may cause significant errors under these conditions, which can be minimized by taking into account the individual nonspecific binding of each serum sample.