Three nasopharyngeal carcinoma (NPC) cell lines (CNE-1, CNE-2 and NPC/HK-1), two squamous cell carcinoma (SCC) cell lines (T2/CUHK and PWH-S1) and six head and neck cancer specimens (NPC [n = 4], SCC tongue [n = 1] and a thyroid cancer [n = 1]) were incubated with interferon (IFN)-alpha (5 x 10(4) iu/mL) and/or 13-cis retinoic acid (13RA; 10(-5) mol/L) for two days at 37 degrees C. In vitro chemosensitivity was measured using MTT assay. Mild growth inhibition of the five cell lines by IFN-alpha ranged from 7.1% to 51.8% (mean: 18.5%), whereas with 13RA it was zero to 19.7% (mean: 7%). Greater inhibition (14.8-51.0%, mean: 31.8%) was achieved when the two drugs were used in combination. Growth inhibition of the six surgical specimens ranged from 6.9% to 21% (mean: 13.6%) with IFN-alpha; zero to 10.3% (mean: 6.0%) with 13RA; and 6.6-26.5% (mean: 17.7%) when the two agents were combined. Four of the 11 samples showed synergistic antitumour effect when IFN-alpha and 13RA were combined, and six showed subadditive effect. The results show that IFN-alpha and 13RA have a mild in vitro antitumour effect on head and neck cancer cells, and the drug synergistic effect demonstrated in this study suggests that the two agents should be used in combination in clinical application.