Medium conditioned by a highly enriched population of gonadotrophs, cultured as reaggregates in the presence of 0.01 nM GnRH, was concentrated, separated on a reversed-phase HPLC column, and tested for activity on lactotroph development in pituitary reaggregate cell cultures of 14-day-old rats. The number of cells expressing prolactin (PRL) mRNA was estimated by image analysis after in situ hybridization of paraffin-embedded sections. The number of these cells entering the mitotic cycle was estimated by autoradiography of [3H]thymidine ([3H]T) incorporation. One HPLC column fraction expanded the section area occupied by PRL mRNA cells without displaying an effect on [3H]T labeling of these cells, indicating that this fraction induces differentiation in the lactotroph lineage. The latter fraction was further purified on a second reversed-phase HPLC column, a gel filtration column, and a final reversed-phase HPLC column. From the last column, four substances were isolated that all selectively induced differentiation of lactotrophs. Each of them had an N-terminal amino acid sequence identical to the N-terminal domain of rat proopiomelanocortin (POMC). As determined by mass spectrometric analysis, the M(r)s were 10,091, 10,289, 12,238, and 12,247 Da, respectively. The C-terminal extension of these compounds is possibly up to Gln74 for the former two compounds and up to Gly95 for the latter two. Authentic purified human POMC(1-76) mimicked the effects of the purified 10- and 12-kDa rat POMC fragments. The present data suggest that certain isoforms of rat POMC(1-74) and human POMC(1-76) can stimulate lactotroph growth through a differentiation-inducing action on progenitor cells.