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

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