Northern blot analysis revealed that a differentiation-defective variant (DD-1) of MM14 mouse myoblasts has seven times the prostaglandin endoperoxide synthase mRNA than the parental MM14 myoblasts. There was an even greater increase in the level of prostaglandin endoperoxide synthase protein in the DD-1 cells as compared to that in the MM14 myoblasts. In fact, prostaglandin endoperoxide synthase was not detectable by Western blot analysis of extracts from MM14 myoblasts. Since prostaglandin endoperoxide synthase has been reported to be a gene whose expression is induced transiently, i.e., growth-regulated, upon mitogen stimulation of quiescent cells, the RNA abundance of other growth-regulated genes was examined including: KC, JE, c-myc, 1B6, and vimentin. Northern blot analysis revealed that the mRNA abundance of JE, KC, and c-myc is 12-, 17-, and 2-fold higher, respectively, in growing DD-1 cells than in growing MM14 myoblasts. In contrast, there was little difference in the mRNA abundance of 1B6 and vimentin. These results are consistent with the hypothesis that increases in the levels of expression of prostaglandin endoperoxide synthase and some growth-regulated genes are integral to the expression of the differentiation-defective phenotype and may in fact contribute to this phenotype.