A biologically-active succinylated porcine relaxin was iodinated by a modification of the Bolton-Hunter method. Fibroblasts cultured from the mouse pubic symphysis and human skin were used to investigate relaxin binding sites. 125I-labelled relaxin binding to both cell types was time- and temperature-dependent. An accelerated rate of labelled hormone degradation (90%) was observed when both cell types were incubated at 37 degrees C. Specific relaxin binding sites on the mouse and human cells were observed as other peptides, such as insulin, epidermal growth factor, glucagon, hFSH and human prolactin, failed to inhibit relaxin binding. Further results indicate that porcine relaxin is mitogenic to these specific fibroblasts because increasing concentrations (10(-9) to 10(-6) M) of this hormone stimulated cell growth in vitro. These data suggest that the effect of relaxin at the target tissue level is mitotic in nature.