Cytochemical staining of monocyte‐specific esterase (MSE) is widely used for identification of the monocytic lineage in leukaemias. Deficiency of this enzymatic activity occurs as a familial trait and the deficiency has been shown to occur with greater frequency in patients with lymphoproliferative or gastrointestinal malignant neoplastic diseases than in normal blood donors. Reverse transcriptase polymerase chain reaction (RT‐PCR), sequencing and quantification by Northern blot analysis was conducted on the MSE mRNA of 12 subjects with monocyte esterase deficiency (MED) and seven MSE‐positive subjects to examine whether mutations were present or whether the defect was quantitative. Mutations were not found in the mRNA sequences. However, MED subjects had significantly less MSE mRNA than MSE‐positive subjects (P = 0·001). These findings show that deficiency of monocyte esterase activity in MED is not as a result of the presence of inactive isoenzymes and may be owing to an abnormality in the regulation of mRNA production.