The membrane labeling of monocytes by monoclonal antibodies directed against platelet glycoproteins Ib (AN51), IIb (Tab), IIIa (C17), IIb-IIIa complex (J15) and to antigens common to platelets and monocytes (anti-monocyte platelet antigen and FA6 152) has been investigated by an ultrastructural immunogold method. Only with FA6 152, which identifies a structure shared by erythroblasts, platelets, and monocytes, was labeling obtained on membranes of both platelets and monocytes from normal blood. With all the other monoclonal antibodies, platelets were highly labeled but monocytes lacked quantitatively significant label; however, focal microparticles which exhibited gold particles were adherent to the membrane of monocytes. This localized labeling, interpreted as resulting from the fragmentation of platelet membranes during monocyte isolation with adhesion of the fragments to monocyte surfaces, was verified by two approaches. First, double staining with C17 visualized by an anti-IgG coupled to 40 nm gold particles and MO2 recognizing exclusively a surface monocyte antigen, as visualized by an anti-IgG coupled to 15 nm gold particles, was performed. The absence of colocalization of large and small gold particles either on monocytes or on microparticles confirmed the exclusive cell origin. Second, when analyzed by quantitative x-ray analysis, monocyte associated gold following C17 treatment was restricted to platelet pseudopods and fragments on whole mount spread cells. Finally, when monocytes were spread immediately after blood collection in the absence of sedimentation and centrifugation to prevent platelet activation, platelet rosetting was avoided and the number of microparticles markedly decreased. Thus, the attachment to monocyte membranes of microparticles originating from platelets may be confused with true labeling of monocytes by antibodies to platelet glycoproteins if analysis is limited to immunofluorescence.