Satellitism of platelets to monocytes was observed in 3 patients suffering from polycythaemia vera, a myeloproliferative disorder and chronic lymphocytic leukaemia. This phenomenon occurred on cells in the buffy coat obtained from venous blood anticoagulated with heparin. The existence of platelet satellitism was reported to occur mainly to polymorphonuclears, but the present observation, as well as a previous report from our department, indicated that it may also exist to monocytes. Satellitism was achieved either by adherence of the platelets to monocytes, giving the impression of rosette formation, or by fusion of platelet pseudopodia with the monocyte membrane. The importance of this phenomenon in the evaluation of the platelet count using automatic instruments is discussed.