Previous research suggested that the beta-lyase-mediated production of a monomethylated selenium metabolite from Se-methylselenocysteine is a key step in cancer chemoprevention by this agent. In an attempt to affirm the concept, the present study was designed to evaluate the activity of methylseleninic acid, a compound that represents a simplified version of Se-methylselenocysteine without the amino acid moiety, thereby obviating the need for beta-lyase action. The in vitro experiments showed that methylseleninic acid was more potent than Se-methylselenocysteine in inhibiting cell accumulation and inducing apoptosis in TM12 (wild-type p53) and TM2H (nonfunctional p53) mouse mammary hyperplastic epithelial cells, and these effects were not attributable to DNA damage, as determined by the comet assay. In general, methylseleninic acid produced a more robust response at one-tenth the concentration of Se-methylselenocysteine. It is possible that these cell lines may have only a modest ability to generate a monomethylated selenium species from Se-methylselenocysteine via the beta-lyase enzyme. In contrast, methylseleninic acid already serves as a preformed active monomethylated metabolite, and this could be an underlying reason why methylseleninic acid acts more rapidly and exerts a more powerful effect than Se-methylselenocysteine in vitro. Interestingly, the distinction between these two compounds disappeared in vivo, where their cancer chemopreventive efficacies were found to be very similar to each other [in both methylnitrosourea and dimethylbenz(a)anthracene rat mammary tumor models]. The beta-lyase enzyme is present in many tissues; thus, animals have an ample capacity to metabolize Se-methylselenocysteine systemically. Therefore, Se-methylselenocysteine would be expected to behave like methylseleninic acid if beta-lyase is no longer a limiting factor. Taken together, the present in vitro and in vivo results provide strong evidence in support of our earlier hypothesis that a monomethylated selenium metabolite is important for cancer chemoprevention. Methylseleninic acid could be an excellent tool, especially for molecular mechanism studies in cell culture, and some of these attributes are discussed.