Fifty sections showing cervical intra-epithelial neoplasia grade III were stained with mucicarmine and periodic acid-Schiff reagents to demonstrate mucin production. Seven of these showed mucin within the neoplastic cells (14%); these cases were considered to be a form of adenosquamous carcinoma in situ and lend support to the hypothesis that invasive adenosquamous carcinomas arise from an indifferent reserve cell.