Pasteurella multocida toxin induces localized osteolysis in the turbinate bones of swine. Osteolysis appears to be due to an increased level of osteoclastic bone resorption, although osteoblast activity may also be impaired. We studied the effects of purified toxin on the osteoblastic phenotype of the ROS 17/2.8 rat osteoblastic osteosarcoma cell line. Treatment of both embryonic bovine lung cells and a nonosteoblastic rat osteosarcoma cell line (ROS 25/1) with nanomolar doses of toxin produced marked cytotoxic actions. In the osteoblastic ROS 17/2.8 cells, this level of toxin reduced expression of an osteoblastic marker (alkaline phosphatase), was associated with matrix mineralization, but had no cytopathologic action. The osteoblastic cell population may be resistant to a direct cytotoxic effect but is nevertheless a target for toxin action.