A study of fine-needle aspiration biopsies (FNAB) in six cases of medullary carcinoma of thyroid (MCT) revealed that the presence of spindle cells in addition to the more usually recognized polygonal cells is a common feature and a significant clue to the diagnosis. Both spindle and polygonal cells contain cytoplasmic granules that stain positively for calcitonin by the immunoperoxidase technic. Amyloid, when present, can be identified presumptively in the Papanicolaou stain. This can be confirmed by restaining with Congo red. Both immunoperoxidase staining for calcitonin and amyloid staining for Congo red work well on the decolorized Papanicolaou-stained smears.