The presence of argyrophil and argentaffin cells and epithelial mucosubstances was studied in 187 mucinous ovarian cystadenomas of varying malignancy. Sixty-two per cent of the benign tumours contained these cells. The corresponding figures for tumours of borderline malignancy and malignant tumours were 60 and 51, respectively. The epithelial cells of the mucinous cystadenomas characteristically contained periodic acid-Schiff reactive and diastase resistant secretory material, mixed with acid mucin which contained both sulphate and carboxyl groups. The amount of mucin diminished with increasing malignancy, as did the ratio of neutral to acid mucins. Mucin histochemistry in tumours with and without argyrophil cells in a given group of malignancy was similar, and the ages of the patients having those tumours did not differ significantly.