The incidence of chromosome aberrations was studied by peripheral blood incubation (52 hr, the last 4 hr in the presence of Colcemid) using a modification of the Evans' technique in twenty-two men exposed to either vapors of metallic mercury (Group I) or organic mercury (Group II). Mercury concentrations of the work areas frequently exceeded the Maximum Allowable Concentrations in the past. During the year proceding the investigation, mercury values ranged between 0.15 and 0.44 mg/m3. None of the investigated men were poisoned, but all had had repeated increased mercury absorption with urinary eliminations reaching 890 microgram/l and 896 microgram/l for subjects in Groups I and II, respectively. The incidence of chromosome aberrations was significantly higher (P less than .001) in subjects exposed to mercury as compared with a control group (ten subjects) of a similar mean age. There was no statistical difference in the incidence of chromosome aberrations between men belonging to Groups I and II. Although an increase of both chromatid gaps and breaks was noticed in exposed men, the incidence did not significantly differ from controls. No chromatid interchanges were recorded and no difference between exposed and control subjects was noticed concerning the frequency of aneuploid or polyploid cells.