The identity of the neutral cholesteryl ester hydrolase (CEH) in human monocyte/macrophages is uncertain. Prior studies indicate that hormone sensitive lipase (HSL) is a major CEH in mouse macrophages, and that HSL mRNA is present in human THP-1 monocytes. In the present study, HSL mRNA expression was examined in THP-1 cells as a function of differentiation status and cholesterol enrichment. By RT-PCR with primer pairs that span exon boundaries, HSL mRNA was demonstrated in THP-1 monocytes and phorbol-ester differentiated THP-1 macrophages. cDNA identities were confirmed by sequencing. By Northern blotting, with HSL cDNA as probe, THP-1 monocytes were found to contain HSL mRNA of approximately 3 and 3.9 kb. In THP-1 macrophages, the 3 kb mRNA was greatly diminished, while the level of the 3.9 kb mRNA was maintained. mRNA of approximately 3 and 3.9 kb are those expected of the 86-kDa (adipocyte) and 117-kDa (testicular) HSL isoforms, respectively. The presence of the testicular isoform mRNA was confirmed in THP-1 cells by amplification and sequencing of an isoform-specific cDNA. Additionally, Northern-blot comparisons showed that the 3 and 3.9 kb mRNA in THP-1 comigrated with the HSL mRNA in 3T3-L1 adipocytes and rat testis, respectively. The level of the 3.9 kb mRNA did not vary greatly with cholesterol enrichment. Thus, the HSL gene is transcribed in THP-1 cells both before and after differentiation into macrophages; after differentiation, the predominant mRNA is that for the 117-kDa isoform. This isoform is a CEH, and may mediate some CE turnover in THP-1 cells.