The activities of several lysosomal hydrolases including beta-glucuronidase, acid phosphatase and hexosaminidase were compared in serum from 19 well-nourished subjects and 13 children (age 5--24 months) who were suffering from marasmus. The marasmic children exhibited growth retardation and muscle wastage but had normal serum protein values and absence of psychomotor retardation or oedema. Significant changes were observed in serum beta-glucuronidase and acid phosphatase activities. Compared to the control group, serum beta-glucuronidase (determined at pH 4.5 using the fluorogenic substrate, 4-methylumbelliferyl-beta-D-glucuronide) was 2.3-fold higher (p less than 0.001) in the marasmic children. In contrast, serum acid phosphatase values were approximately 50% lower (p less than 0.01) in the marasmic population. Serum hexosaminidase values in the two groups under study were not significantly different. Determination of the beta-glucuronidase to acid phosphatase ratio permitted effective discrimination (p less than 0.001) of serum from normal and protein-calorie malnourished children. The finding that the elevated value of the beta-glucuronidase : acid phosphatase ratio (0.64--1.37) decreased to within the normal range of values (0.10--0.43) after nutritional rehabilitation of several marasmic cases indicates that the determination of serum lysosomal hydrolases using fluorogenic substrates might provide a rapid and sensitive quantitative method for objectively evaluating the status of protein-calorie malnourished children and their responsiveness to nutritional therapy.