The pathogenicity of 13 strains of Bacillus licheniformis was studied in immunodepressed mice. The strains had been isolated from cases of bovine abortions (n = 5), bovine feedstuffs (n = 3), soil (n = 1), and grain products (n = 2). The origin of two strains was unknown. Groups of 10 mice were inoculated intravenously with B. licheniformis bacteria at doses from < 10(6) to 10(10) colony-forming units. Following 7 days of infection, the animals were euthanized and examined bacteriologically, histologically, and immunohistochemically using a PAP technique based on primary polyclonal rabbit anti-B. licheniformis antibodies. B. licheniformis bacteria were reisolated from the liver, spleen or kidneys of mice in all groups. Inflammatory lesions were present in mice of all immunodepressed groups, but only brain and pulmonic lesions were definitely attributed to B. licheniformis infection, as strong immunostaining was found within these lesions. It is concluded that all strains of B. licheniformis examined were pathogenic for immunodepressed mice, and that spontaneous infections may be established by bacterial strains to which susceptible individuals are accidentally exposed.