Mevalonic aciduria was found in a 19-month-old boy with severe failure to thrive and developmental delay (Sweetman et al., 1985; Hoffmann et al., 1986). Cardinal manifestations included recurrent anaemia, mild hepatosplenomegaly, bilateral central cataracts and dysmorphic features. Serum cholesterol was low (1.82.1mmol/L). Urinary organic ~acids were normal except for a large elevation of mevalonic acid, an intermediate in cholesterol biosynthesis. The activity of meval0nate kinase (ATP: mevalonate-5-phosphotransferase, EC 2.7.1.36), the enzyme catalysing the second committed step in cholesterol biosynthesis, was severely deficient in the patient's fibroblasts, lymphocytes and lymphoblasts. We have developed an optimized assay for mevalonate kinase in extracts of cultured human fibroblasts utilizing [2-14C]mevalonate as substrate. The enzymatic products, [2-14C]mevalonate-5-phosphate and [2-14C]mevalonate-5-pyrophosphate, were separated and quantified by cellulose thin-layer chromatography. With heterozygote detection as a goal, we undertook a study of family members of the patient with mevalonic aciduria. This report presents the activities of mevalonate kinase in lysates of lymphocytes isolated from whole blood of these individuals and of controls.