Distinct antigenic determinants were identified on native molecules of human growth hormone (hGH) and chorionic somatomammotropin (hCS) on the basis of competitive inhibition assays with eight murine monoclonal antibodies. Effective competition for antigen binding within a pair of antibodies indicated overlapping combining site specificities whereas a lack of competition suggested binding to sterically distinct structural moieties. An antigenic determinant, specific for hGH was detected by antibodies QA68 and NA27, whilst another marginally hCS-cross-reactive site was bound by NA71. Two distinct determinants fully expressed by either hGH or hCS were bound by antibody pairs NA39/EB2 and EB1/EB3 respectively, whereas a single hCS-specific determinant was recognized by antibody EB4. An unexpected reciprocal cross-inhibition of soluble antigen antibody complex binding was observed between antibodies reacting to distinct determinants, i.e. for NA27 towards NA39/EB2 and for NA71 towards EB1/EB3. These results were tentatively interpreted in terms of conformational changes of antigen when bound in soluble immune complexes, but an alternative explanation of steric hindrance cannot yet be excluded. The effect of monoclonal antibodies on the hormonal biological activity was investigated in a dose response study of the hormone-dependent growth stimulation of NB2 lymphoma cells in tissue culture. Although all eight antibodies were specifically growth-inhibitory, major quantitative differences in their efficacies have been observed. At limiting hormone doses antibodies EB2 NA39 were most effective whereas QA68 and NA71 were the most potent at excess hormone input. Various mechanisms operating through inhibition of hormone binding and/or modulation of cell receptor-bound complexes have been considered.