Previous reports have suggested that dimethyl sulfoxide (DMSO) may be a useful reversible G1 arresting agent for synchronizing Raji Burkitt's lymphoma cells (K. Takase et al. (1992) Cell Growth Differ. 3, 515-521; M. Sawai et al. (1990) Exp. Cell Res. 187, 4-10). We have therefore critically evaluated several aspects of DMSO's effects using Daudi and Ramos Burkitt's lymphoma (BL) cells. In BL cells starved in the presence or absence of DMSO for 4 to 6 days (approximately four to six doubling times), the following observations were noted: (A) Both Daudi and Ramos cells show increased cell synchrony accompanied by apoptosis when starved in RPMI 1640 supplemented with 10% fetal calf serum (FCS). Inclusion of 1.5% DMSO causes a diminution in apoptosis with minimal effects on synchrony. (B) Lowering the FCS concentration to 5% induces apoptosis. DMSO-mediated protection from apoptosis is observed in Daudi but not in Ramos. (C) When human serum (10%) is used instead of FCS, Daudi cells show no apoptosis and DMSO is without effect on cell cycle distribution. By contrast, Ramos cells show significant apoptosis, which is prevented by the inclusion of DMSO. (D) When starved in a chemically defined medium (AIM-V), both Daudi and Ramos cells show significant apoptosis. DMSO protects Ramos from apoptosis under these conditions. (E) Upon removal of DMSO, both Daudi and Ramos cells reenter the cell cycle but with significant apoptosis. (F) The protective effect of DMSO from apoptosis is observed in a narrow range of concentration between 1 and 2%. At higher concentration, DMSO itself induces apoptosis. These results suggest that DMSO itself prevents apoptosis, an effect which may present as an apparent effect on cell synchrony.