HCS-2/8 is a stable human chondrosarcoma cell line with many chondrocytic characteristics and has the capacity to form chondrosarcomas in nude mice. The cells display both biochemically and morphologically definable changes in sparse, subconfluent, confluent and over-confluent phases of in vitro culture. Such features of HCS-2/8 cells may reflect the processes of both proliferation and differentiation of chondrocytes in vivo. We examined the correlations of these changes of HCS-2/8 cells with their transcript levels of 21 proto-oncogenes by Northern analysis. We found no detectable transcripts of 9 proto-oncogenes (c-sis, c-met, c-src, c-lyn, c-fgr, c-ros, c-pim, Blym and N-myc), but detected transcripts of 12 other proto-oncogenes (int-2, erbB, c-abl, c-raf-1, c-fyn, K-ras, H-ras, c-mos, c-myc, c-myb, c-fos, and c-jun). In the over-confluent phase, the levels of c-fos and c-raf-1 were increased several dozen times and about 5 times, respectively, while the level of c-abl was about 1/5th of that in the sparse, subconfluent and confluent phases of culture. The level of int-2 increased about 10-fold in the confluent and over-confluent phases of in vitro culture. The transcript levels of c-mos and K-ras were high in the sparse phase, low in the subconfluent and confluent phases and high in the over-confluent phase. The levels of the other 6 proto-oncogenes in HCS-2/8 cells were constant in all phases of in vitro culture.