We have recently demonstrated that prolactin is able to maintain the level of in receptors in cultured rat hepatocytes. This effect could be modulated by various inhibitors of cellular functions. We report here that an antibody developed against a partially purified prolactin receptor preparation can mimic this effect of the hormone (although to a lesser extent) and that drugs can modulate it in a similar manner. In particular, cycloheximide (50 micrograms/mL), which reduced basal receptor levels by approximately 40%, totally reversed the maintenance induced by the antireceptor serum. Actinomycin D (10 micrograms/mL), another protein synthesis inhibitor (at the transcriptional level), had no effect of basal receptor concentration, but counteracted by about one-half the antiserum-induced maintenance. This effect of actinomycin D is much clearer here than the effect previously observed on prolactin-induced receptor levels in rat liver cells in culture. The effect of dinitrophenol (1 mM) on basal levels was of limited amplitude but maintenance was again partly reversed by this drug. In accordance with previous results obtained with prolactin, chloroquine (100 microM) and colchicine (1 microM) failed to alter prolactin binding either in the absence or presence of 5% antireceptor serum. The effect of the antiserum indicates that prolactin itself is not required beyond the membrane for its effect on receptor regulation to be attained. These results also confirm our previous results with prolactin maintenance of prolactin receptor levels in rat liver cells in culture, that the mechanism of receptor maintenance appears to be due in part to a stimulation of receptor synthesis but to be independent of the internalization or of lysosomal degradation.