Possible effects of an ex vivo fluorescent labeling procedure of blood platelets on the platelet viability were examined by different function tests. The degree of the impairment of the platelet function proved to be principally correlated with the dye concentration used during the labeling procedure. However, the physiological properties of the labeled platelets obviously remain unrestricted by using intraplatelet fluorescein isothiocyanate (FITC) concentrations below 0.55 mg/1011 platelets, corresponding to 8.5 × 106 FITC molecules bound to one platelet. A procedure for fluorescent labeling of blood platelets is described which is considered to be preferably suitable to prepare platelets for vital microscopic platelet observations