PC3 (pheochromocytoma cell-3) is an immediate early gene isolated as sequence induced in the rat PC12 cell line during neuronal differentiation by nerve growth factor (NGF). PC3, which is expressed in vivo in the neuroblast when it ceases proliferating and differentiates into a neuron, has partial homology with two antiproliferative genes, BTG1 and Tob. Here we report that overexpression of PC3 in NIH3T3 and PC12 cells leads to marked inhibition of cell proliferation. In stable NIH3T3 clones expressing PC3, the transition from G1 to S phase was impaired, whereas the retinoblastoma (RB) protein was detected as multiple isoforms of M(r) 105,000-115,000 (indicative of a hyperphosphorylated state) only in low-density cultures. Such findings are consistent with a condition of growth inhibition. Thus, PC3 might be a negative regulator of cell proliferation, possibly acting as a transducer of factors influencing cell growth and/or differentiation, such as NGF, by a RB-dependent pathway. This is the first evidence of a NGF-inducible immediate early gene displaying antiproliferative activity.