Human monocyte/macrophage serine esterase (HMSE) can be distinguished from esterases of other blood cells by a specific isoelectric focusing pattern of five enzyme variants. Using a HMSE-1 cDNA, expression of mRNA was investigated by nonradioactive in situ hybridization of human biopsy specimens and cytospin preparations. HMSE-1 transcripts could be detected in the myelomonocytic cell line U-937, blood monocytes and in tissue macrophages. Immune accessory cells of the monocyte/macrophage lineage did not express detectable amounts of HMSE-1 mRNA. These results indicate that HMSE-1 represents a cell-lineage-specific enzyme which can be used to characterize physiological and neoplastic variants of the human monocyte/macrophage cell system. The method described will enable further insight into the physiological function of HMSE-1.