An immunoradiometric assay (IRMA) of human thyrotropin (hTSH), based on magnetic solid phase separation, was studied especially in terms of its nonspecific bindings (B0) which were identified as a product of the interaction between an altered form of radioiodinated anti-hTSH monoclonal antibody (125I-mAB) and the uncoupled magnetizable cellulose particle (matrix). Preincubation with the same matrix, solid phase saturation with milk proteins, tracer storage at 4 degrees C and serum addition during incubation were found to be particularly effective in preventing their formation. These findings were used to reproducibly decrease nonspecific bindings to values < 0.1% (or < 70 cpm), thus increasing the signal-to-noise ratio (B60/B0) up to values of 300-500. This way hTSH radioassays were obtained with functional sensitivities of about 0.05 mIU/L and analytical sensitivities of the order of 0.02 mIU/L. Such sensitivities, and, more importantly, a general improvement in assay performance, were obtained in a highly reproducible manner and all over the useful tracer life.