A 4.1 kb mouse whey acidic protein (mWAP) promoter was cloned from a C57BL/6 cosmid library. The tissue-specific and developmental pattern of expression of a hybrid gene comprised of the mWAP promoter fragment and the human protein C (HPC) gene was analysed in transgenic mice. The corresponding RNA was detected mainly in the mammary gland, with 'leakage' of expression in the salivary gland and kidney. The developmental pattern of transgene expression differed from that of the endogenous WAP gene. In particular, recombinant HPC (rHPC) transcripts were detected earlier in pregnancy than WAP RNA, with no significant increase during lactation. This indicates that regulatory elements responsible for developmental regulation are located outside the 4.1 kb mWAP gene promoter fragment, or if present, may be subject to position effects. Precocious expression of the transgene did not compromise the health or nursing abilities of transgenic females. Expression of rHPC affected the appearance of the mammary alveoli and alveolar epithelial cells in lactating transgenic mice. The alveoli were less distended and alveolar epithelial cells appeared cuboidal with centrally positioned nuclei. We suggest that the inefficient intracellular processing of rHPC can alter the histological appearance of alveolar epithelial cells in the transgenic mammary gland.