Serum concentrations of lactate dehydrogenase (LDH), alpha-hydroxybutyrate dehydrogenase (HBD), phosphohexose isomerase (PHI), leucine aminopeptidase (LAP), beta-glucuronidase (beta-gluc) and angiotensin-converting enzyme (ACE), were measured in 107 cases of untreated acute myeloid leukaemia which were classified by morphological, immunological and cytochemical criteria. The results show that serum LDH was increased in most cases, irrespective of leukaemic subtype, serum PHI and LAP were significantly higher in monocytic variants and serum beta-gluc and ACE levels were generally within normal limits. In addition, significant (p less than 0.05) relationships were found between serum LDH, PHI, LAP and beta-gluc concentrations and the numbers of circulating leucocytes.