We have studied formalin fixed, paraffin-wax embedded tissue from 35 medulloblastomas, collected over 23 years (27 non-desmoplastic and 8 desmoplastic) with KP1 and Mac387 two monoclonal anti-monocytes/macrophage (M/Ms) antibodies recommended for use on paraffin wax embedded tissue. In non-desmoplastic medulloblastomas, outside areas of necrosis, M/Ms were detected in 50% of cases with KP1 and 52% with Mac387. M/Ms were seen in 100% of desmoplastic medulloblastomas with both antibodies. Semiquantitative assessment revealed that, on average, desmoplastic tumors had at least three times as many infiltrating M/Ms as non-desmoplastic tumors. There was no significant difference in the findings with the two antibodies or, between recently embedded and "older" tumors. The findings may be indicative of the presence of a host M/Ms immune response in medulloblastoma, which may be more accentuated in desmoplastic medulloblastomas. Furthermore, we conclude that these antibodies are quite suitable for the study of infiltrating M/Ms, thus lessening (but not obviating) the need for frozen tissue for immunohistological studies.