Using Southern blot analysis of DNA from mouse-hamster somatic cell hybrids, we have mapped Lmyc and Bmyc, two members of the myc family of genes, to mouse chromosomes 4 and 2, respectively. Furthermore, we have compared the regulation of Lmyc and Bmyc expression under different growth conditions and during in vitro differentiation of the murine EC line F9 and considered the findings in relation to our previous studies on Nmyc and c-myc expression in the same line (Sejersen et al., 1987). Lmyc was down-regulated at an early stage of visceral endoderm differentiation, similarly to c-myc and Nmyc, while Bmyc was expressed at a constant low level at all stages. Lmyc, but not c-myc and Nmyc, was upregulated in terminally differentiated visceral endoderm cells. Inhibition of protein synthesis by cycloheximide for 4 h induced a 70% increase in Lmyc and 30% increase in Bmyc transcript levels, indicating that the expression of these genes is negatively regulated by a short-lived protein. Mitogenic stimulation with insulin and transferrin did not affect Lmyc and Bmyc mRNA levels. Lmyc transcripts have a half life of 30 min, whereas the Bmyc transcript is highly stable, with a half life of 6 h. The half-lives of the c-myc and Nmyc transcripts have been estimated previously as 40 and 130 min, respectively.