Protozoa possess a wealth of purine-salvage enzymes, many with unique, or unusual, substrate specificities. As a result, many opportunities for the chemotherapist exist. An exemplification is found in the conversion in schistosomes of allopurinol ribonucleoside to the corresponding ribonucleotide followed by further anabolism to the very toxic 4-aminopyrazolo(3,4-d)pyrimidine 1-ribonucleotide. The same organisms convert another inosine analog, formycin B, to the ribonucleotide, but its inhibitory effects appear to be exercised primarily by inhibition of the organism's adenylosuccinate synthase. A substantial segment of the Phylum Protozoa shows no vestigial traces of ability to synthesize purines de novo although thymidylate synthase appears to be present in many. The absence of other tetrahydrofolate catalyzed reactions suggests that these functions were never acquired.