Human anti-murine antibody (HAMA) response is a serious problem in the repeated infusion of murine monoclonal antibodies (MoAbs). HAMA positive sera were obtained from seven patients with colorectal cancer, pancreas cancer, malignant melanoma or myocardial infarction who had previously received radiolabelled MoAbs. The nature of HAMA was analysed using size exclusion high performance liquid chromatography (HPLC) after incubating with radiolabelled MoAbs including IgG, Fab or human/mouse chimeric Abs. Immune complexes composed of HAMA and MoAbs were formed. The percentage of radioactivity with a high molecular weight was related to HAMA levels determined by enzyme linked immunosorbent assay. Most radioactivity present in immune complex shifted to the antibody fraction after the addition of normal murine serum. All of seven sera were reactive with all four murine IgGs and this suggests that HAMA in these patients recognised the constant region of MoAbs. In one patient, HAMA was considered to recognise the variable region and to be anti-idiotypic. There was no significant binding with human/mouse chimeric Abs in any HAMA positive serum, although five out of seven patients were reactive with murine MoAb Fab, indicating that HAMA was composed of Abs responsive to the CH1 or CL region of murine IgG. These results suggest that (1) HAMA was composed of Ab responsive to Fc portion and/or CH1 or CL region of murine IgG, and (2) human/mouse chimeric Abs look promising in the repeated infusion of MoAb in HAMA positive patients.