Leishmania donovani promastigotes contain intense tartrate-resistant cell surface acid phosphatase (ACP1) which blocks superoxide anion production by activated human neutrophils [A.T. Remaley et al. (1984) J. Biol. Chem, 259, 11173-11175]. An extensively purified preparation of ACP1 dephosphorylates several phosphoproteins which are phosphorylated at serine residues; these include: pyruvate kinase (Km 1.6 microM; Vmax 71.4 U (mg protein)-1), phosphorylase kinase (Km 0.076 microM; Vmax 5.4 U (mg protein)-1) and histones (Km 4.86 microM; Vmax 2.2 U (mg protein)-1). However, the specific activity of the leishmanial phosphatase on these phosphoproteins is very low as compared to other phosphoprotein phosphatases. The phosphatase activity of ACP1 was also low on phosphohistone phosphorylated at tyrosine residues. Phosphatidylinositol-4,5-diphosphate (PIP2) and inositoltriphosphate (IP3) were also tested as ACP1 substrates. PIP2 was hydrolyzed rapidly by ACP1. The rate of hydrolysis of PIP2 was higher at pH 6.8 (Km 2.35 microM; Vmax 107 X 10(3) U (mg protein)-1) than at pH 5.5 (Km 4.16 microM; Vmax 71 X 10(3) U (mg protein)-1). 32P-labeled IP3 was also a substrate for ACP1; the hydrolysis products consisted of a mixture of inositoldiphosphate and inositolmonophosphate. ACP1 and ten other phosphatases were tested for their ability to dephosphorylate proteins and to inhibit O2- production by stimulated human neutrophils. There was no correlation between the protein phosphatase activity of the acid- and alkaline phosphatases and their ability to block neutrophil O2- production. The results indicate that ACP1 probably blocks the production of reduced oxygen intermediates by a mechanism that does not involve dephosphorylation of phosphoproteins; however, the possibility that the parasite's phosphatase affects phagocyte metabolism by degrading PIP2 or IP3 should be considered.