SummaryA case of acute monobiastic leukaemia well documented by light microscopy and classical cytochemistry (Myeloperoxidase, naphthol AS-D chloroacetate, alpha naphthyl acetate, naphthol AS-D acetate esterase with Na F Inhibition) is reported.The ultrastructural study permits us to classify the blast cells as promonocytes rather than monoblasts. The presence of ribosome-lamella complexes with a positive peroxidase reaction in about 90 % of the infiltrating cells is very striking.The authors point out the relationship between high levels of muramidase in serum and urine, the demonstration of muramidase inside the blasts by immunofluorescence and immunoperoxidase techniques, and the evolution of the leukaemia.The ribosome-lamella complexes are described in a few rare cases of monobiastic leukaemia but without muramidase activity as in the present report.