A monoclonal antibody (Ki-M6) against the CD 68 antigen, which labels cells of the monocyte/macrophage system, was tested on Bouin-fixed, paraffin-embedded samples of normal, reactive and neoplastic tissues by an avidin-biotin-peroxidase complex method, with the aim of establishing its value in diagnostic pathology. In normal human tissues, Ki-M6 reactivity was confined to the so-called resident macrophages populating normal organs under physiological conditions. Moreover, restricted reactivity against cells of macrophage lineage was observed in reactive and inflammatory lesions. Granulocytes, monocyte/macrophage-related immune accessory cells, and other analysed normal tissue structures did not reveal any reactivity. Ki-M6 was strongly reactive with the cases of benign (4/4) and malignant (15/15) fibrous histiocytomas, in addition to the true histiocytic lymphomas (3/3). Cases of granular cell tumour (2/3) showed strong reactivity with Ki-M6, whereas only few immunoreactive cells, with weak staining, were seen in the other Ki-M6-positive neoplasms [neurofibroma (3/3), benign schwannoma (1/2), ganglioneuroma (1/1), malignant schwannoma (5/9), melanoma (9/28), dermatofibrosarcoma protuberans (1/1), myelomonocytic leukaemia (3/3)]. Among the epithelial malignancies tested (47 cases), Ki-M6 was positive only in renal cell carcinoma (11/14). Malignant lymphomas of the Hodgkin (56 cases) and non-Hodgkin type (67 cases) were uniformly non-reactive. From these data, Ki-M6 appears to be an excellent marker of monocyte/macrophage-related cells and appears to be a reliable indicator for fibrous histiocytomas and true histiocytic malignancies. The availability of this additional antibody capable of staining routinely processed tissue is of practical interest.