We have localized a fraction of the enzyme, purine nucleoside phosphorylase (PNP), to the centrioles and basal bodies of mammalian, avian, and protozoan cells. Two completely independent methods were used, one based on the ultrastructural cytochemistry of the enzyme activity and one based on immunofluorescence microscopy using an antibody raised in rabbit against purified human PNP. PNP catalyzes the reversible conversion of purine nucleosides and inorganic phosphate to the corresponding purine bases and ribose-1-phosphate. Its partial localization to centrioles and basal bodies raises the possibility that purine compounds are involved in centriole replication and/or in the regulation of microtubule assembly in vivo. No centriolar PNP could be detected in primary skin fibroblast from two infants with severe immunodeficiency disease associated with the absence of soluble PNP. This raises the possibility that defects in centriole function may contribute to the impaired division and maturation of T lymphoid precursor in this inherited disorder. Initially, the immunofluorescence analyses were complicated by a residual centriole-binding antibody that persisted in immunoglobulins from immune animals after complete removal of anti-PNP by affinity chromatography. Binding was abolished by exposure of cells to sodium periodate, indicating that this (and possibly other) "spontaneous" anticentriole antibodies in rabbit serum may be directed against carbohydrates.