Acid sphingomyelinase activity was determined in Epstein-Barr virus-transformed lymphoid cell lines (LCL) established from patients affected with Niemann- Pick disease (NPD) using several substrates: sphingomyelin derivatives, radiolabeled [14C]sphingomyelin (SM), fluorescent 7V-(10-(l-pyrene)decanoyl)sphingomyelin (P10-SM) or colored trinitrophenylaminolaurylsphingomyelin, and the chromogenic non-natural substrate 2-/V-(hexadecanoyl)amino-4-nitrophenylphosphoryl-choline. LCL from NPD Type A and Type B showed a severe deficiency of acid sphingomyelinase determined using either substrate, whereas LCL from normal subjects had an activity close to that of blood leukocytes. Sphingomyelinase in normal LCL had the same pH optimum (5.0-5.2) and molecular form (pi 5.8) as the enzyme from other sources; identical profiles and activity levels were obtained using the various analogues of sphingomyelin. However, among these derivatives, the assay using P10-SM appeared as the most useful and sensitive for enzymatic diagnosis of NPD. Electron microscopy of NPD LCL demonstrated the lysosomal storage. These results prove the validity of LCL as an experimental model system for NPD.