Three experiments were carried out on 53-hr cultured human leukocytes on chromosome mutagenicity of benzene with particular reference to combined effects with r-radiation and inhibitory effect on rejoining of radiation-induced chromosome breaks. The results and their analyses were summarized as follows :
1) Benzene can induce mainly chromatid-type deletions, especially gaps, suggesting that the cells in their late S-G2 stage have a higher susceptibility to chromosome breakage by benzene.
2) The aberration yield of dicentrics and rings induced by 100 rads irradiation can significantly be enhanced by the treatment of benzene equal to or in excess of 0.2mM.
3) Quantitative analyses using newly defined “Synergetic effect factor” revealed that combined cytogenetic effect of benzene with radiation could be synergetic exclusively in dicentrics and rings, being almost additive in the other types of aberrations.
4) The experiment with dosage fractionation method showed that the more strongly benzene could inhibit the rejoining of radiation-induced chromosome breaks, the higher concentration it was treated at.
Related with these results, possible induction mechanisms were discussed with reference to a significance of induced chromatid gaps, and the repair and its inhibition of radiation-induced DNA-strand (s) breaks. Further studies for related effects by benzene are needed on protein synthesis, activities of repair enzymes and fine structure of chromosomes.