The effects of some high-polar compounds on the cytomorphological characteristics of cultured cells of synovial and osteogenic human sarcomas (Sa-2 and Sa-4) were studied. Incubation of Sa-2 cells with 1 or 2% dimethylsulfoxide (DMSO) or 100 mM dimethylformamide (DMF) during 4-6 days induced the cellular, structural and functional changes in Sa-2 cells which were considered as manifestations of pseudoepithelial differentiation. DMSO did not influence the activity of Sa-2 cell enzymes, while DMF suppressed their activity, mainly that of acid and alkaline phosphatases. DMSO and DMF induced in Sa-4 cultured the appearance of the cells which were similar cytomorphologically to normal osteoblasts. DMSO and DMF significantly depressed the activity of acid phosphatase in Sa-4 cells and transformed the positive reaction of alkaline phosphatase to the negative one. DMSO and DMF prolonged the time of the doubling of Sa-2 and Sa-4 cells. The effects of DMSO and DMF were reversible. Methylformamide (200 mM) and dimethyl acetamide (0,1 and 10 mM) did not induce analogous changes in human sarcoma cells.