A cell-mediated cytotoxicity assay was used to evaluate the specific reactivity of patients with squamous cell carcinoma to an established cell line derived from squamous cell carcinoma of the cervix. Heat inactivated sera from patients with squamous cell carcinoma could very effectively nullify this specific cytotoxicity (P less than 0.01). There was no significant difference between the cytotoxic blocking activity of the sera from patients with different stages of the cancer in this experimental setup.