The prolactin (PRL)-like activity released into synthetic culture medium from concanavalin-A (Con-A)-stimulated mouse splenocytes was bioassayed using 3H thymidine incorporation into Nb2 lymphoma cells. At low cell density (1.0 x 10(6) cells/ml medium) Con-A-stimulated splenocytes from Balb/c mice released more PRL-like activity than did splenocytes from normal C3H/HeJ mice. This difference was maintained with animals of either sex, and for animals of different ages (1, 3 and 5 months). The strain difference was observed both when splenocytes were co-cultured with Nb2 cells, and when postcultured medium from splenocytes was tested. Con-A-stimulated splenocytes also released a substance into the medium which was inhibitory to PRL-induced Nb2 cell proliferation. The concentration of the inhibitory substance in the medium was related to the number of splenocytes in culture and was similar in splenocyte media preparations from both strains of mice. Polyclonal antisera to bovine PRL, rat PRL, mouse placental lactogen II, and mouse PRL did not neutralize the effect of Con-A-stimulated splenocytes to induce Nb2 cell proliferation. Bioassay of the partially purified PRL-like activity found in postcultured medium indicated that the dose-dependent induction of Nb2 cell proliferation was parallel with that of purified mPRL. Using two mouse PRL antisera (Sinha and Talamantes #118) in Western blot analyses, no antibody binding was observed to the partially-purified splenocyte PRL-like material. The data from the antisera neutralization experiments and the SDS/PAGE Western blot analyses clearly indicated that the PRL-like activity produced by Con-A-stimulated splenocytes has little homology with pituitary PRL. Further studies are required to establish the precise molecular identity of the PRL-like activity from Con-A-stimulated splenocytes.