Activities of certain acid hydrolases (viz. acid phosphatase, beta-glucosidase, beta-galactosidase, N-acetyl-beta-D-glucosaminidase and cathepsine) of post mitochondrial fraction of liver and spleen were studied during the course of Dipetalonema viteae infection in Mastomys natalensis. The values are significantly higher from prepatent to patent phase of infection as compared with normal animals. However, a decrease in the activity of hepatic acid phosphatase and N-acetyl-beta-D-glucosaminidase was noticed in latent phase of infection while a several fold increase in the activity of these enzymes was observed in splenic tissue when there were no detectable microfilariae (mf) in peripheral circulation. The results suggest that lysosomal acid hydrolases which constitute an important component of resistance may be activated by mf products through the sensitized cells of RE system.