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Actinomycosis infection of bone is rare and its diagnosis challenging. Here, we aim to identify and verify its microstructural features and the potential value for differential diagnosis. We investigated the dry preparation of the lumbar vertebrae and pelvic ring of a purported case of actinomycosis documented by a post-mortem examination in 1891. Macroscopic inspection, conventional radiology, μCT, 3D reconstruction, and histological examination were employed. All approaches revealed new periosteal bone deposition with increased vascularisation of the os coxa, vertebrae, and sacrum. The μCT revealed cortical loss underneath the new bone formation; the 3D reconstruction and histological examination revealed plexiform bone and granular structures. The plexiform bone is the result of reactive rapid growth and remodelling processes, and is consistent with pathomorphological findings summarised in the autopsy report (soft tissue abscesses and formation of fistulas caused by " Actinomycosis intestine et ossis ilei sin. "). This is the first case of a historically documented case of actinomycosis infection investigated by μCT and histology. Different degrees of tissue damage and inflammatory reaction in form of plexiform bone, which has not been reported previously, was identified. The noted bone tissue modifications are not solely pathognomic of actinomycosis; they characterise other diseases, as well. Histological evaluation is not appropriate for identifying the aetiology of the granular structures observed here; but clinically such aggregations appear in tissue affected by actinomycosis. Histochemical and molecular-genetic analyses are obligatory to affirm the diagnosis based on micromorphological features. [ABSTRACT FROM AUTHOR]