Chromosome aberration tests on cultured peripheral blood lymphocytes were carried out in two groups of open-cast mining workers engaged in stripping operations (Group I) and digging operations (Group II), in a group of coal cleaning plant workers used as a positive control, and in a group of nonexposed subjects serving as matching controls. Peripheral blood samples in group I and group II miners were collected in March and June of 1984, the antimutagenic potential of long-term prophylactic vitamin administrations (1 g of ascorbic acid given daily for a period of 3 months and 100 mg of vitamin A a day for two 5-day cycles) initiated after the first blood sampling was tested in group II miners. The frequency of aberrant cells (% AB.C.) found in the cultured lymphocytes of group I miners was increased at both times of measurement (2.41% AB.C. and 2.29% AB.C.), in group II miners given the vitamin prophylaxis it dropped significantly (P greater than 0.01) from the initial 3.16% AB.C. in March to 1.77% AB.C. in June, and in the group of coal cleaning plant workers (1.30% AB.C.) it was more or less equal to that of matching controls (1.21% AB.C.). Implicitly, coal dust alone can hardly be made responsible for the rise in % AB.C. It appears that the increase in frequency of chromosome aberrations observed in soft coal open-cast mining workers rather reflects an increased risk of genetic injury due to exposure to ever-present fumes and fire hazards that may give rise to the mutagenic chemicals belonging to a class of polycyclic aromatic hydrocarbons. The antimutagenic effect of ascorbic acid prophylaxis was characterized by a significant drop in % AB.C. and a speed-up of the lymphocyte cell cycle expressed as the proliferating rate index (PRI) whose value rose from the original 1.41 to 1.69, thus reaching the PRI value of matching controls (1.63).