Human blood platelets were found to carry the complete pathway of de novo purine nucleotide synthesis. The rate of purine synthesis was gauged by the rate of incorporation of precursor (14C)formate into purines. The effect on formate incorporation of several compounds known to inhibit purine synthesis de novo was studied. Adenine, orotic acid and azaserine inhibited purine synthesis, but hypoxanthine and allopurinol did not. Platelet content of phosphoribosylpyrophosphate (PRPP) and of ribose-5-phosphate (R-5-P) was found to be similar to that in human blood leukocytes and erythrocytes. Incubation of intact platelets with high inorganic phosphate concentrations caused an increase in platelet PRPP content but did not affect R-5-P content or the rate of purine synthesis de novo.