The expression of Fc-IgG and C3b membrane receptors by granulocytes and their precursors was examined in 78 patients with primary myeloid dysplasia (MDS). The marrows were categorized into five morphological groups, broadly corresponding to those described by the FAB group, and further graded into mild and severe according to the severity of dysgranulopoiesis. Fractionated bone marrow and peripheral blood granulocyte receptors were assessed by rosette formation with optimally sensitized ox erythrocytes and the results compared with those found in 17 normal marrows and with those previously reported in megaloblastic anaemia. Fc-IgG and C3b receptor expression was generally increased in maturing granulocytes in all MDS diagnostic groups but premature expression was particularly marked in chronic myelomonocytic leukaemia and refractory cytopenia. Receptor patterns showed a better correlation with the severity of dysgranulopoiesis than with the morphological types. It is suggested that, as in megaloblastic anaemia, changes in membrane receptor expression reflect nuclear-cytoplasmic asynchrony. The results further indicate considerable immunological heterogeneity of granulocytes within individual MDS categories.