An immunohistological study of paraffin wax embedded tissue from three cases of plasmacytoid monocyte neoplasms, using a panel of antibodies which react with fixation resistant leucocyte markers, is reported. This neoplasm was found to have a distinctive antigenic profile, being negative for CD3 and elastase, but positive for CD43 and CD68. This immunological phenotype, coupled with its characteristic morphological features, should facilitate the recognition of this rare neoplasm in routinely processed tissue. Furthermore, the term "plasmacytoid monocyte sarcoma" is proposed to designate it because it is inappropriate to refer to it as a lymphoma. As all cases have been associated with a myeloproliferative disorder (usually an acute or chronic myeloid leukaemia), these tumours probably represent the accumulation in lymphoid tissue of neoplastic cells which have differentiated along the plasmacytoid monocyte pathway.