The preparation of sections of bone marrow cores in a routine histology laboratory requires decalcification and paraffin embedding, which produces shrinkage and considerable loss of cellular detail. This may be avoided by using plastic embedding procedures. This report describes a simplified routine procedure for using methylmethacrylate as a plastic embedding medium for the preparation of semi-thin sections of undecalcified bone marrow cores. A modification of the May-Grunwald-Giemsa stain is also given which provides good colour differentiation of various haematopoietic cells in the marrow. The method is simple, reproducible, requires no expensive equipment, and is suitable for routine processing of bone marrow biopsy cores in any histopathology laboratory.