The nonspecific accumulation of radioactivity in bone marrow, liver, and spleen in patients with melanoma injected with radiolabeled whole IgG of anti-high molecular weight-melanoma associated antigen (HMW-MAA) monoclonal antibody (mAb) hampers the application of immunoscintigraphy to visualize melanoma lesions. This nonspecific background can be reduced by utilizing fragments which do not contain the Fc portion of anti-HMW-MAA mAb. Since the in vivo targeting to melanoma lesions of radiolabeled F(ab')2 and Fab' fragments of anti-HMW-MAA mAb is critically dependent on their binding parameters, we have analyzed the effect of fragmentation on the binding characteristics of the anti-HMW-MAA mAbs 225.28, 763.74, and TP41.2. The three mAbs recognize distinct and spatially distant determinants with a heterogeneous distribution on the pool of HMW-MAA molecules synthesized by melanoma cells. The determinant recognized by mAb TP41.2 is detectable on a markedly smaller population of HMW-MAA molecules than those recognized by mAbs 225.28 and 763.74. 125I-labeled F(ab')2 fragments of the three mAbs displayed an immunoreactive fraction similar to that of the whole IgG, while Fab' fragments displayed a lower one. Fragmentation of mAbs 225.28 and TP41.2 to F(ab')2 produced a 2- and 1.5-fold reduction in their association constants but did not cause a significant change in that of mAb 763.74. Cleavage of F(ab')2 fragments to Fab' fragments produced 2-, 40-, and 7-fold reductions in the association constants of mAbs 225.28, 763.74, and TP41.2, respectively. These changes qualitatively fit the predictions of theory for univalent and bivalent mAb binding, since mAbs 763.74 and TP41.2 appear to show bivalent binding to melanoma cells and mAb 225.28 to show univalent binding. The affinity constant of IgG, F(ab')2 and Fab' fragments of mAbs 225.28, 763.74, and TP41.2 displays an inverse relationship with the extent of their time-dependent release from the membrane of melanoma cells. Since no endocytosis of mAb could be detected, the latter results suggest that radioactivity remains bound to melanoma cells in vivo for a longer time following injection of F(ab')2 fragments than following that of Fab' fragments of each of the anti-HMW-MAA mAb tested. Radiolabeled Fab' fragments of mAbs 763.74 and TP41.2 displayed a marked reduction in their reactivity with some of the antiidiotypic mAb tested. The loss of some idiotopes is likely to be caused by changes in the conformation of the molecules associated with the fragmentation of IgG and by damage during the iodination procedure.