Secretory immunoglobulin A (SIgA) is a major component of the secretory immune system and has been demonstrated in the sera of patients with various pathologic conditions. Current studies were undertaken to quantitate secretory components (SC) in sera of patients with squamous cell carcinoma in the head and neck region by means of a sensitive double-antibody radioimmunoassay. Patients with head and neck carcinoma were found to have threefold higher levels of SC (1.54 +/- 0.74 mg/ml) compared to normal controls (0.51 +/- 0.22 mg/ml, P less than .0001). Of interest was that the sera of patients who had successfully been treated 2 to 6 months prior to the study demonstrated statistically lower levels than those of patients with untreated carcinoma or recurrent disease. SC in sera appears to be in the form of SIgA. The origin and diagnostic significance of SC are briefly discussed.