We have purified a cDNA fragment complementary to the mRNA coding for one of the major urinary proteins (MUPs) synthesized in the mouse liver. Using this cDNA as a hybridization probe, we have shown that the level of MUP mRNA is lower in the livers of females and castrated males than in those of males. The addition of testosterone to females and castrated males results in an increase in the concentration of the mRNA to levels found in males. There are approximately 15 genes per haploid genome coding for the MUPs; this allows a possible new interpretation of some of the genetic data concerning the regulation of levels of the different MUPs in the urine (Szoka and Paigen, 1978). Finally, we have shown that mouse MUP and rat cr2.-globulin mRNA share common sequences, but that there are surprising differences in gene number and regulation of the genes in these two closely related animals.