The in vitro study of muscle cell growth is hampered by the presence of non-muscle cells, particularly fibroblasts. The heterobifunctional cross-linking agent, N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP) has been used to create a novel "toxic growth factor" to address the problem. Epidermal growth factor (EGF), which stimulates fibroblast but not satellite cell proliferation, was conjugated via SPDP to a potent ribosome inhibitor, pokeweed antiviral protein (PAP). By preferentially binding to fibroblasts, it was hoped that EGF-PAP could cytotoxically eliminate fibroblasts from primary cultures of rat skeletal muscle satellite cells. While EGF-PAP did prove to be a fibroblast cytotoxin, it could not completely eliminate them from cell cultures. Low dose-time exposures improved the ratio of multinucleated cells to mononucleated cells (percent fusion) by up to 66% over controls, but increased concentrations, or durations of EGF-PAP treatment, proved detrimental to satellite cell growth and/or differentiation.