Although hCG and mouse epidermal growth factor activate different signaling systems in a clonal strain of Leydig tumor cells (designated MA-10), both compounds are capable of increasing steroidogenesis, decreasing transcription of the LH/CG receptor gene, and attenuating adenylyl cyclase. We have recently identified a 21-kilodalton protein (p21) that is a potential intermediary in the actions of mouse epidermal growth factor and hCG. Characterization of p21 revealed similarities to stathmin, a ubiquitous phosphoprotein whose phosphorylation is associated with diverse second messenger pathways. Using two different experimental approaches, Western blotting and immunoprecipitation, we present data that identify p21 as stathmin.