We have introduced expression constructs for mouse PRL (mPRL) or a nonsecreted form of mPRL into the PRL-responsive Nb2 rat lymphoma cell line. Cell lines resulting from transfection of Nb2 cells with the wild type mPRL construct synthesize and secrete mPRL. These cells are able to grow independently of added lactogens, and conditioned media and cell extracts from these cultures stimulate the growth of Nb2 cells. In contrast, cells synthesizing the nonsecreted mPRL do not proliferate in the absence of added lactogenic hormones, and conditioned media from these cell cultures do not have PRL-like activity in the Nb2 cell growth assay. PRL protein is detected in these nonsecreting cell lines; however, extracts from these lines are generally unable to stimulate Nb2 cell proliferation. These results indicate that cells can respond in an autocrine fashion to PRL, but that an intracellular form of PRL is unable to activate Nb2 cell growth.