We previously derived, from a nonmalignant clonal line of rat myogenic cells (L6 alpha 1), two sublines which have lost the capacity to differentiate, the M4 cell of low malignancy and the RMS4 cell of high malignancy. In the present study it is shown that 14 of 15 protooncogenes analyzed exhibit detectable levels of transcripts during L6 alpha 1 cell proliferation. When L6 alpha 1 cells from myotubes, the levels of c-abl, c-myb, and c-Ha-ras transcripts remain unchanged, the level of c-N-ras RNA is augmented, the level of c-erbB RNA is markedly reduced, and all other c-onc transcripts (c-erbA, c-sis, c-src, c-fes, c-fms, c-fos, c-myc, c-Ki-ras, and the putative tyrosine kinase transcript of the c-fgr gene) become hardly, if at all, detectable. Surprisingly, when the three cell types are growing at similar rates only, one protooncogene (c-mos) is not expressed at detectable levels in L6 alpha 1, two others (c-fos, c-erbA) are not expressed in M4 or in RMS4, and three additional ones (c-erbB, c-sis, c-src) are expressed in M4 but not in RMS4. Moreover the level of c-fes RNAs is markedly lower in RMS4 than in M4 or L6 alpha 1. By contrast, the level of two c-Ki-ras 5.4- and 2.2-kilobase transcripts is lower in M4 and L6 alpha 1 than in RMS4, and the latter contains another abundant c-Ki-ras 3.8-kilobase transcript which is hardly detectable in M4 and not at all in L6 alpha 1. These data suggest an activation of the c-Ki-ras gene in the malignant myoblasts and some relationship between the progression of malignancy and inactivation of certain other c-onc genes.