The pulmonary toxicity of short fiber asbestos (1332214) preparations was studied in rats. Wistar-rats were injected intratracheally with 40 milligrams (mg) chrysotile (12001295), crocidolite (12001295), tremolite (14567738), or actinolite (77536664), given in two 20mg doses 1 month apart. At least 90% of the fibers had lengths of 3 to 5 microns. Selected rats were killed after 2, 4, 6, 12, or 18 months and examined. Other rats were maintained for lifespan observation. At death they were necropsied, the examination focussing on pulmonary tumors. All animals gained weight normally. Lung weights of all asbestos exposed rats were significantly increased. The largest increase occurred in chrysotile treated rats, followed by tremolite, actinolite, and crocidolite treated rats in that order. The maximum increases in lung weight induced by chrysotile and actinolite occurred at 6 months. Maximum increases induced by crocidolite and tremolite occurred at 12 months. Changes in lung collagen content showed similar patterns. All preparations produced pulmonary fibrosis. The most severe fibrosis was observed in chrysotile treated rats. Bronchiolar and alveolar epithelial hyperplasia were occasionally seen in crocidolite and chrysotile treated rats. Three of 38 chrysotile treated rats, two of 39 crocidolite treated rats, one of 40 tremolite treated rats and one of 35 actinolite treated rats developed pulmonary tumors. The first tumor developed after 15 months in a chrysotile treated rat. The other tumors developed after 21 to 25 months. The tumors were squamous cell carcinomas, fibrosarcomas, or adenocarcinomas. No tumors developed in the controls. The authors conclude that when developing hygienic standards for asbestos the effects of short fibers must be considered.