Incorporation of pyrimidine ribonucleotides in Giardia intestinalis occurs via uracil phosphoribosyltransferase (UPRTase). The enzyme was purified over 1000-fold to apparent homogeneity from parasite extracts, using Fast Protein Liquid Chromatography, namely Mono Q anion exchange, Mono P chromatofocusing and Superose 12 chromatography. The specific activity of the purified enzyme was 3100 nmol min-1 mg protein-1. The enzyme was found to be a dimer of mol. wt. 76,000. Kinetic analysis, including initial velocity and product inhibition studies, indicated that it obeyed a rapid-random equilibrium mechanism. GTP and dGTP caused a dramatic increase in the activity of the enzyme, though there was no effect on the Michaelis constants. All other nucleotides tested were without effect or were inhibitory. The effect of GTP is similar to that observed for UPRTase from E. coli but not from other eukaryotes.