Neutrophils from patients with chronic myeloid leukaemia (CML) have an aberrant expression of leukotriene (LT)C4 synthase. In order to learn more about the regulation of this abnormality, LTC4 synthase mRNA expression was determined by reverse transcription polymerase chain reaction. A digoxigenin (DIG)-labelled LTC4 synthase RNA was synthesized and incubated in cytosolic extracts from CML neutrophils, normal neutrophils and eosinophils. LTC4 synthase mRNA was detected in total but not cytoplasmic RNA from normal neutrophils. In contrast, LTC4 synthase mRNA was found in the cytoplasm of CML neutrophils and in normal eosinophils, which also express the enzyme. The DIG-labelled LTC4 synthase RNA was, as opposed to normal neutrophils, degraded in cytosolic extracts from CML neutrophils. The degradation was time dependent and cell concentration dependent. Degradation was also seen in eosinophils, indicating that degradation of LTC4 synthase RNA was correlated to the expression of the protein. This study showed that the difference in expression of LTC4 synthase in normal and CML neutrophils was not because of a total lack of LTC4 synthase mRNA in normal neutrophils. However normal neutrophils lack, in contrast to CML neutrophils, LTC4 synthase mRNA in the cytoplasm. This discrepancy is not caused by a stabilized LTC4 synthase RNA in the cytosol of CML neutrophils. Instead an abnormal degradation of LTC4 synthase RNA was found in the cytosol of CML neutrophils.