The presence of an estrogen-regulated protein (24K) in normal, dysplastic, metaplastic, and neoplastic cervical epithelium correlates with histologic criteria for squamous cell maturation. The 24K protein, originally discovered in the MCF-7 breast cancer cell line, was studied in 51 cases by the modified immunoperoxidase Avidin-Biotin Complex method, using an anti-24K mouse monoclonal antibody. Immunostained sections were compared to hematoxylin-and-eosin-stained sections cut from the same tissue block. The 24K protein was observed to be located primarily in the parabasal or "prickle cell layer" of normal cervical tissue (6 of 7 normal cervical tissue specimens tested were positive for 24K protein. Specimens were obtained from surgery for nonneoplastic causes) and in all cases (12 of 12) of dysplasia and carcinoma in situ. Intercellular bridges of these cells showed prominent immunostaining in normal cervix and dysplasia. 24K protein was observed as a granular cytoplasmic stain in all cases of squamous metaplasia (5 of 5) and keratinizing squamous cell carcinoma (9 of 9), and in 8 of 14 cases of nonkeratinizing squamous cell carcinoma. In this latter group, immunostaining was confined to only those cells showing cytoplasmic eosinophilia on H&E sections. In no case was the presence of the 24K protein associated with areas of mature keratin. 24K immunostaining was also observed in the reserve cells of morphologically normal endocervical glands adjacent to areas of dysplasia and carcinoma. We conclude that 24K protein is associated with squamous cell maturation and may be an important marker of reserve cell hyperplasia and squamous metaplasia.