Abstract Sodium selenite (Na 2Se0 3) was tested for its sister-chromatid exchange (SCE)-inducing ability in human whole blood cultures and for the effect of its co-exposure with methyl methanesulfonate (MMS) or N-hydroxy-2-acetyl aminofluorene (N-OH-AAF) on SCE frequency. Long exposure times (77 h and 96 h) to 3.95 × 10 -6 M Na 2SeO 3 resulted in cell death as measured by mitotic indices, but mitotic figures were present after exposure to higher concentrations for a shorter time (19 h). High Na 2SeO 3 concentrations (7.90 × 10 −6 and 1.19 × 10 −5 M) resulted in a three-fold increase in the SCE frequency above background level (6–7 SCEs/cell). Exposure of lymphocytes to 1 × 10 −4 M MMS for the last 19 h of culture yielded an average SCE frequency of 30.17 ± 0.75 while a similar exposure to 2.7 × 10 −5 M N-OH-AAF resulted in 13.61 ± 0.43 SCEs/cell. Simultaneous addition of the high Na 2Se0 3 concentrations and MMS or N-OH-AAF to the cultures resulted in SCE frequencies that were 25–30% and 11–17%, respectively, below the sum of the SCE frequencies produced by the individual compounds.