The present study was undertaken to obtain more precise information about the purine biosynthetic pathway in human blood cells. 5'phosphoribosyl-l-pyrophosphate (PP-ribose-P) amidotransferase was found in cell-free extracts from all leukemic cells and normal lymphocytes and therefore these cells could synthesize the first intermediate of the purine-de-novo-synthesis. Normal leucocytes, erythrocytes and bone marrow cells lack this enzyme system and have an absolute requirement for externally supplied purines via salvage pathway. Leukemic blast cells show different enzyme activities independent of their cell count. Kinetic studies with the crude enzymes showed sigmoidal substrate velocity curves for PP-ribose-P, whereas glutamine shows hyperbolic kinetics. The leukemic cell enzymes from all four donor types (ALL, CLL, AML and CML) are rapidly saturated with low concentrations of PP-ribose-P and less inhibited by the physiological feedback inhibitor, adenosine 5'monophosphate. The crude enzymes of normal spleen lymphocytes and leukemic cells were further purified (10 to 15-fold) and substrate velocity curves for PP-ribose-P and glutamine show now hyperbolic kinetics and double reciprocal plots were linear with and apparent Km for PP-ribose-P of 0.14 mM and for glutamine 2.0 mM. In the presence of different concentrations of AMP, the PP-ribose-P substrate velocity plot changed from a hyperbolic to a sigmoidal curve; no difference in the degree of the inhibition between both partially purified enzymes (normal spleen lymphocytes and leukemic cells from all four donor types) could now be observed.