A pilot trial of enzyme replacement using splenic and plasma forms of alpha-galactosidase A was undertaken in 2 brothers with Fabry disease, an X-linked glycosphingolipid storage disease. Partially purified preparations of alpha-galactosidase A from human spleen and plasma Cohn fraction IV-1 were prepared aseptically for in vivo administration. The disappearance of enzymatic activity from plasma, levels of circulating substrate, and potential immune response were evaluated following IV administration of 6 unentrapped doses (2,000 U/kg) of each enzyme form to the respective recipient during a 117-day period. Repeated injections were well tolerated. The circulating half-life of the splenic form was about 10 min whereas that for the plasma form was approximately 70 min. No immune response was detected by skin and immunodiffusion tests or by alterations in the maximal activity or clearance kinetics for either enzyme following successive administrations. After each dose of the splenic form, the concentration of the accumulated circulating substrate globotriaosylceramide, decreased maximally (approximately 50% of initial values) in 15 min and returned to preinfusion levels by 2-3 hr. In marked contrast, injection of the plasma form decreased the circulating substrate levels 50-70% by 2-6 hr; the concentrations of globotriaosylceramide gradually returned to preinfusion values by 36-72 hr. Two consecutive doses of the plasma form, administered on days 1 and 3, reduced the circulating substrate concentration to normal levels. Prior to the 6th enzyme administration, circulating substrate was stable-isotope labeled by the infusion of dideutero-glucose, and the effects of each enzyme form on circulating substrate degradation and reaccumulation were determined. The results of this study indicated that labeled (newly synthesized) substrate reaccumulated following injection of the splenic enzyme whereas both unlabeled (previously stored?) and labeled substrate reaccumulated in the circulation after administration of the plasma form. These studies demonstrated the differential disappearance kinetics of the splenic and plasma forms of alpha-galactosidase A, their differential effects on circulating substrate degradation and reaccumulation, as well as the lack of an immune response to repeated administrations of these homologous, unentrapped enzymes.