The monoclonal IgM 3G5, which reacts with the surface membranes of rat insulinoma cells RINm5F, was purified by HPLC and labeled with 125I using Protag-125; bovine IgG (bIgG) was similarly radiolabeled, and used as a control. 125I-3G5 was incubated with RINm5F cells either at 4 degrees C or at 37 degrees C. 125I-3G5 bound onto RINm5F cells growing in Petri dishes remained approx. constant over 44 h when incubated at 4 degrees C, whereas at 37 degrees C radioactivity was released back in the medium starting at 3 h (plateau at approx. 20 h). At the end of incubation at 37 degrees C, activity in the medium included a high percentage of free 125I (15.69 vs 2.62% for bIgG, and 1% for 3G5 at 4 degrees C). In a cell suspension experiment, cell-bound 125I-3G5 also remained constant over a 24 h incubation at 4 degrees C, whereas at 37 degrees C it decreased to 37.5% of its initial value (64.1% at 4 h). Concomitant microautoradiography showed diffuse radioactive deposits within the RINm5F cells following incubation with 125I-3G5 (but not 125I-bIgG) at 37 degrees C. These results indicate that 3G5-IgM reacts with a surface antigen on the RINm5F cells, but is rapidly internalized by the cells: within the cells, this antibody undergoes some metabolic processing which results in the release of free 125I outside the cells.