The cocarcinogenicity of foundry particulates was studied in hamsters. Male Syrian-Golden-hamsters were administered respirable size particles of commercial silica (7631869), olivine (1317711), zircon (14940682), chromite sands, aluminum silicates, ferric-oxide (1309371), or a reference quartz (minusil) plus 0 or 3 milligrams benzo(a)pyrene (50328) (BaP) weekly for 15 weeks by intratracheal instillation. Positive controls received ferric-oxide plus BaP or saline plus BaP. Negative controls received saline. The animals were observed for clinical signs and tumor development. Survival was recorded. Moribund and all surviving animals were killed 92 weeks after the first treatment, and tissues were examined for histopathological changes. Survival was decreased in hamsters given minusil without BaP and in 6 of 12 groups receiving particulates plus BaP. Wheezing, dyspnea and polypnea were the most frequently observed clinical signs. These were not correlated with survival or pathological findings. Pulmonary tumors occurred in every group given BaP. Except for animals receiving aluminum-(60)-silicate, the tumor incidences were greater than the saline plus BaP, but not different from the ferric-oxide plus BaP treated animals. Squamous cell and adenomatous carcinomas and adenocarcinomas of the lung and bronchi were the most frequently seen tumors. Mean latencies for tumor development were 60 to 70 weeks. No pulmonary fibrosis was observed; however, significant increases in granulomatous inflammation were observed in animals administered silica, minusil, and aluminum-(48)-silicate. Hyperplasia and tumors of the stomach were also observed. The authors conclude that silica sand and silica sand substitutes are not carcinogenic in the hamster respiratory tract. When instilled with BaP, silica sand and silica sand substitutes show a definite cocarcinogenic response. No fibrotic response was induced by silica sand and silica sand substitutes. The granulomatous inflammation seen may be indicative of a fibrotic response in other species. The hamster appears to be a poor model for studying fibrotic responses.