The effects of lactogenic hormones on protein secretion by 25.5-G4.2.3 cells, a rat mammary myoepithelial cell line immortalised with a temperature-sensitive T-antigen, were investigated. Insulin, prolactin, estradiol and progesterone had no effect but hydrocortisone induced the secretion of two proteins with molecular masses of 175 kDa (p175) and 146 kDa (p146), 10-30-fold and 5-fold respectively. The induction of p175 and p146 synthesis by hydrocortisone was greater at 39.5 degrees C than at 33 degrees C reflecting the increased differentiation of 25.5-G4.2.3 cells at the higher temperature. Rat mammary epithelial cells did not synthesise p175. After addition of hydrocortisone to 25.5-G4.2.3 cells, there was a lag phase of 10 h before the synthesis of p175 was induced. Half-maximal induction of p175 synthesis required a hydrocortisone concentration of 0.5 microM. p175 was identified as alpha 2-macroglobulin by N-terminal amino-acid sequence determination and immunoprecipitation with a specific antibody. Hydrocortisone induced a 5-kb alpha 2-macroglobulin-specific mRNA transcript in 25.5-G4.2.3 cells. Myoepithelial cells are responsible for synthesising the basement membrane around the rapidly expanding mammary alveoli during pregnancy. Myoepithelial cells also secrete metalloproteinases which are probably involved in turnover of the basement membrane. We suggest that increased levels of hydrocortisone during pregnancy induce the synthesis of alpha 2-macroglobulin, which is believed to be a potent inhibitor of metalloproteinases, by rat mammary myoepithelial cells to reduce proteolytic degradation of the basement membrane.