OBJECTIVE: To advance understanding of the molecular mechanisms of hormone-inducible, specific gene expression in the breast tissue: In the lactating mammary glands of the mouse (BALB/c), adrenal glucocorticoid influences, specific transcription, and cellular accumulation of casein mRNA (mRNAcsn), measured by molecular hybridization with a specific cDNA probe (cDNAcsn). Alveolar morphogenesis during culture of the whole mammary gland in a glucocorticoid-free medium fails to support mRNAcsn accumulation in presence of prolactin. Subsequent incubation in medium containing both prolactin and cortisol stimulates mRNAcsn. Medium containing prolactin and corticosteroids can induce simultaneous alveolar morphogenesis and mRNAcsn accumulation in the gland in vitro. The alveolar gland fails to elicit mRNAcsn in medium with prolactin or cortisol alone, indicating that neither hormone alone can induce casein gene expression. Preincubation with cortisol followed by prolactin elicits mRNAcsn and this is due to the action of prolactin and residual cortisol in the explant, but preincubation with prolactin followed by cortisol fails to do so presumably due to the short half-life of the peptide hormone. As analyzed by the cDNAcsn probe expression of the casein gene in different preneoplastic mammary hyperplasias exhibit an altered hormone response. No expression of the casein gene is measurable in the mammary tumors produced by the different mammary hyperplasias, regardless of the endogenous hormone environment of the animals. Whether prolactin or cortisol may act at discrete level of transcriptional and post-transcriptional control, and the altered hormone responses of the transformed tissue may reflect failure of such control needs to be determined. For this purpose the "casein gene" (derived from cDNAcsn) was cloned in E. coli strain X 1776, using the plasmid PBR322 as a vector. Sufficient amount of "casein DNA" obtained from the clone should provide the probe for DNA-excess hybridization assay and these should facilitate studies on transcriptional and post-transcriptional control.