The chromosomal aberration (CA), sister chromatid exchange (SCE) and micronucleus test (MN) were employed to investigate the in vitro effect of antimicrobial food additive benzoic acid on human chromosomes. Lymphocytes were incubated with various concentrations (50, 100, 200 and 500 mu g/mL) of benzoic acid. The results of used assays showed that benzoic acid significantly increased the chromosomal aberration, sister chromatid exchange and micronucleus frequency (200 and 500 mu g/mL) without changing the pH of the medium in a dose-dependent manner. Also this additive significantly decreased the mitotic index (MI) at the highest concentration for 24 h and 100, 200 and 500 mu g/mL for 48 h. This decrease was dose-dependent as well. However, it did not effect the replication (RI) and nuclear division (NDI) indices.