Sphingomyelinase activities were assayedin vitroin cultured skin fibroblasts of 61 patients with Niemann‐Pick disease (NPD). Residual activities found in type A and B were 1% and 4%, respectively, of the mean control values, i.e. significantly higher in type B. In 27 cases with NPD type C, the mean activity was 42% of that in controls, with residual activities ranging from 15% up to normal. Fifteen pregnancies at risk for NPD type A and B were monitored; 4 affected foetuses were found.The uptake of exogeneously added radiolabeled sphingomyelin by cultured cells and metabolism of the choline moiety of this lipid were studied in 35 patients with NPD and 14 controls. No difference of uptake between normal and mutant cells was observed. Normally, 77 ± 5% of the radioactivity taken up was converted to phosphatidylcholine after 18 h incubation, compared to 5±2% (n = 7) in NPD type A. A substantially greater hydrolysis (31 ±12%; n = 8) occurred in NPD type B, and the test allowed complete discrimination between these two types. In NPD type C, 16 patients showed an abnormally low rate of intracellular sphingomyelin degradation (48 ± 5%) while 4 others were not distinguishable from controls. There was a correlation (r = 0.76) between the results of thein vitroandin vivoassays, but also between the severity of the clinical symptoms and the impairment in sphingomyelin degradation. For the diagnosis of NPD type C, thein vivotest gave more reproducible and more clearcut results than thein vitro