SCC (squamous cell carcinoma) antigen is a subfraction of tumour antigen TA-4 isolated from cervical squamous cell carcinomas. Serum concentrations of SCC antigen were measured by radioimmunoassay in 382 control subjects, 70 women with cervical intraepithelial neoplasia (CIN), 517 with cervical carcinoma and 203 with other gynaecological carcinomas. Elevated SCC antigen levels (greater than 2.5 ng/ml) were found in 4% of normal controls, in 7% of women with CIN I-III, in 2%-23% with various forms of genital adenocarcinomas, in 55% with primary and in 76% of those with recurrent cervical squamous cell carcinoma. The positivity rate of the antigen was correlated with tumour stage (FIGO) and lymph node involvement of primary cervical squamous cell carcinomas. During long-term follow-up serum levels of SCC antigen were found to be concordant with tumour activity in 74% of cases. Patients with still elevated marker levels after therapy had twice the recurrence rate of women with normal serum values. Routine determination of SCC antigen during follow-up of cervical cancer is recommended.