Actinomycosis, originally classified as a fungus, is now considered a branching bacteria. Although jaw involvement often presents with classic pathognomonic signs, postcranial disease has not been so characterized. Affected bones from individuals diagnosed in life with actinomycosis were macroscopically and radiologically examined for their macroscopic character. The bones were riddled with spheroid, occasionally coalescing defects associated with periosteal reaction. Erosion penetrated cortical bone as readily as through cortical bone or subchondral bone. X-ray revealed circular lesions with a slight sclerotic margin. Actinomycosis apparently has unique features, which should allow it to be distinguished from multiple myeloma (because of presence of reactive new bone formation) and from fungal disease (because of lack of "fronts of resorption" and penetrating spicules). Similarity to fungal infection is especially of interest because of the earlier phylogenetic classification question.