Cytokinetic changes induced by asbestos (1332214) in mesothelial cells were studied in rats. Male Fischer-344-rats were injected intraperitoneally with 0 or 50 milligrams crocidolite (12001284). Selected rats were killed at various times for up to 18 months and the peritoneally cavity was opened and lavaged. Lavage fluid total and differential cellularity was determined. Samples of parietal and visceral peritoneum were taken and examined by light and electron microscopy. The number of mesothelial cells in the DNA synthesizing phase (S-phase) was determined by a flow cytometric technique. Results for rats sampled at 1, 3, 7, 14, or 28 days were described. Crocidolite caused a large increase in inflammatory cells recovered in the lavage fluid 1 to 14 days post exposure. Large numbers of red blood cells were recovered at 1, 3, and 7 days. The number of mesothelial cells increased sharply at 7 days. Crocidolite induced an early, intense acute inflammatory reaction in the mesothelial and submesothelial tissues which was characterized by capillary dilatation, edema, and macrophage and polymorphonuclear cell infiltration. Crocidolite fibers were initially noted on the peritoneal surfaces and on the surfaces of the abdominal organs. At later times they were phagocytized and incorporated into submesothelial cells. Incorporation of fibers into the peritoneum was associated with proliferation of surface and submesothelial cells. Diffuse peritoneal thickening and nodule formation was noted in proliferating submesothelial cells. Collagen was observed in the nodules by 28 days. Crocidolite induced a sharp increase in S-phase synthesis which peaked at 3 days and which was still elevated at 28 days. The authors note that because the results are still preliminary it is not possible to determine whether the changes induced by crocidolite in the mesothelial cell population are specific for asbestos or whether they will progress to neoplasia.