Introduction At the request of the American Petroleum Institute@ Washington@ D.C.@ Hine Incorporated carried out a cytogenetic study in order to further assess the mutagenic potential of three petroleum fractions designated 60 Solvent@ Rubber Solvent@ and High Aromatic Solvent. A previous study of these materials (Project U-150-14 (EA-11) had resulted in equivocal findings@ with bone marrow analyses showing some mutagenic changes possibly attributable to the solvents@ but with blood analyses yielding less than desirable numbers of metaphases for evaluation because of suppression of mitosis in many of the samples. The current project was undertaken to determine whether the mutagenic effects previously observed could be confirmed and to clarify the situation with regard to the blood analyses. There were certain differences in the experimental design of the two studies. Instead of using different groups of rats for the 1-day@ 7-day@ and 30-day post-treatment chromosome analyses as was previously done@ this project utilized the same set of rats by means of serial bone marrow biopsies and blood collections. In addition to allowing the evolution of chromosomal changes to be followed in the same rat@ this procedure permits the addition of demecolcine directly to the bone marrow suspension rather than to the rat@ thus avoiding the toxic effects of this antimitotic agent.