Blood monocyte differentiation to macrophages was examined in nine patients with primary myelodysplastic syndromes using the skin window technique. Emigrated cells were stained cytochemically for acid phosphatase reaction after 1, 2, 4, 7, 9, 12 and 23 h. Compared to age-matched controls, seven patients showed a significant delay in lysosomal enzyme acquisition, which is associated with macrophage differentiation. Our results with this in-vivo assay demonstrate an involvement of the monocyte/macrophage system in primary myelodysplastic syndromes and show that patients often have a disturbance in macrophage differentiation.