Growth factors are known as a family of polypeptides with powerful influences on angiogenesis, tumor growth and wound healing. Transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF) are structurally related peptides which bind to the same receptor, EGF-R, and also exert similar effects. EGF is a natural component of human tears, and ocular disease leads to decreased concentrations in tear fluid. Using a sensitive radioimmunoassay we investigated whether TGF-alpha is also to be considered a natural component of tear fluid and in which concentrations it occurs. All of 46 tear fluid samples from 24 volunteers contained TGF-alpha. The mean concentration was 161.4 pg TGF-alpha/ml (SD 11.6 pg). No statistically significant correlation was found between tear fluid flow and TGF-alpha concentration in the sample. However, the concentration of TGF-alpha in tear fluid decreased significantly with increasing total time of tear fluid collection (P = 0.002). TGF-alpha levels in samples collected from males (n = 16) appeared to be higher (mean 247.0 pg/ml, SD 15.3 pg/ml) than in those from females (n = 30; mean 180.0 pg/ml, SD 8.5 pg/ml; P = 0.05). No correlation was found between the age of the individuals and the concentration of TGF-alpha in their tear fluid. The findings show that TGF-alpha is, like EGF, a constant component of human tear fluid. The dependence of TGF-alpha concentration on tear fluid flow and the physiological importance of its presence for corneal integrity and ocular surface physiology, however, require further investigation.