Expression of the hapten fucosyl-N-acetyllactosamine was correlated with ultrastructural development in human granulocyte precursors using the monoclonal antibody FMC 10 with immunogold techniques. The antigen was detectable from the myeloblast/early neutrophilic promyelocyte stage onwards and was associated with striking development of the rough endoplasmic reticular system. In addition, low levels of labelling were seen on monocytes, eosinophils and some basophil precursors. Contraction and alignment of the cisternae of the rough endoplasmic reticulum during the promyelocyte stage of neutrophilic differentiation gave the appearance of a plasma cell. However, on closer examination it was apparent that true plasma cells did not react with this antibody.