SCC antigen was measured in the serum of 214 patients with benign diseases and in 251 patients with various cancers. With 2.5 micrograms/L as the upper normal limit for serum, values were positive in 2.9% of 69 healthy subjects (I), 29.0% of 214 patients with benign pathologies (II), and 41% of 217 patients with active cancer (III). The highest values in group II were for patients in renal failure (64%) or with lung diseases (40%) or head-and-neck diseases (21.2%). Specificity of SCC increased (91.1%) when we excluded patients in renal failure or with creatinine values greater than 133 mumol/L. In group III, SCC values were abnormal in 57.7% of patients with squamous cell carcinoma, but in only 9.3% of those with other histological types (P less than 0.001). In squamous cell carcinoma of the lung, cervix, or head and neck, SCC values were related to tumor stage, values being highest in patients with metastases.