A study was made of the action of African swine fever virus (ASFV) on the bone marrow of 12 miniature pigs inoculated intramuscularly with the moderately virulent ASFV isolate E75 and killed 2 to 12 days after infection. A sequential description is provided of the histological lesions of the bone marrow in the experimental animals, which developed haemorrhagic lesions from 6 days after inoculation onwards. Immunohistochemical techniques were used to demonstrate the viral protein VP73 and immunoglobulins (IgG and IgM) in formalin-fixed and paraffin wax-embedded samples of bone marrow tissue. The immunohistological results, platelet counts, viraemia, and anti-ASFV immunoglobulin titres all indicated that thrombocytopoiesis impairment by direct viral action plays a role in the progressive thrombocytopenia characteristic of infection by moderately virulent ASFV isolates.