This study was designed to examine the fibrogenic potentials of four coal slags that are being used as substitutes for silica sand in abrasive blasting. Six groups of 100 male Sprague-Dawley rats, including four coal slag groups, a vehicle control, and a positive control for fibrosis (Minusil quartz), were used. Each dust treatment group was given a single 40-mg dose of test agent via intratracheal instillation. Interim sacrifices of 15 animals per group were performed at 2 d, 3 mo, and 6 mo posttreatment, with the terminal sacrifice conducted at 12 mo. Hematoxylin and eosin stained histologic sections were prepared from designated formalin-fixed tissues collected at each necropsy and examined microscopically. Pulmonary silicon analyses were performed for each group at the 2-d and 12-mo sacrifices. Pulmonary function analyses were conducted for each group at the 3-, 6-, and 12-mo sacrifices. Lung hydroxyproline analyses were conducted for 15 animals in each group at the terminal sacrifice. The pulmonary fibrogenic potentials of the four coal slag groups were compared histologically with the Minusil and vehicle controls. A mild to moderate interstitial fibrosis, which was progressive with time, was noted in each of the coal slag groups. However, the coal slag-induced lung fibrosis was much less than that produced by Minusil. Differences in fibrosis among the individual coal slags were relatively minor and certainly not as striking as those between the slags and Minusil. Other data derived from this study, such as lung hydroxyproline content, pulmonary particulate burdens, pulmonary function, and animal body weights, provided further evidence of a reduced toxicity for the coal slags compared to Minusil.