The micronucleus assay was performed in the peripheral lymphocytes of 32 subjects occupationally exposed to vinyl chloride monomer (VCM) divided into two groups according to years of employment. Blood samples were taken in the period from 24 h to 90 days following a transitory exposure to elevated VCM concentrations of 300 ppm due to the technological process. In subjects with a longer period of employment micronucleus frequencies decreased in proportion to the length of the interval after the last exposure to VCM. The results confirm that the micronucleus assay can serve as a suitable indicator of the time elapsed after last exposure to elevated concentrations of environmental mutagen. It can be assumed that duration of employment may contribute to the occurrence of the cumulative effect produced by exposure to elevated VCM concentrations.