Flow cytometric analysis of human platelets from healthy volunteers by high-resolution immunophenotyping accomplished with enzymatic amplification staining revealed the expression of CD34. Assessment of the platelets with five different monoclonal antibodies to the three different classes of CD34 epitopes suggested that platelets express a unique glycoform of the molecule. Expression of CD34 was verified by immunoblotting. Platelet activation resulted in a marked decrease in surface CD34. CD34 expression has significant implications for the function of platelets as well as for the determination of hematopoietic progenitor cells.