Tissues from 12 oral squamous cell carcinomas were investigated for the presence of blood group antigen A. The antigen reactivity in the carcinomas was compared, by titration, to the reactivity of adjacent normal epithelium included in the same specimen. Normal epithelium always reacted positively. In 6 of 12 specimens the malignant cells gave a completely negative reaction. In 5 specimens there was a patchy distribution of positive reaction on cells within the carcinoma. However, these cells possessed less antigen than the normal epithelial cells at the edges of the surgical specimens. In one specimen the majority of cells in the carcinoma showed no decrease in reactivity. In three cases exhibiting slight epithelial atypia adjacent to the tumour, the reactivity of blood group substances in these areas was found to be higher than in the carcinomas but lower than in the normal mucosa.