ABSTRACT The objective of this study was to determine the effect of ionising and ultraviolet radiation on stimulating melanin secretion in Tetrahymena pyriformis. Cell cultures were exposed to different doses of radiation for 48, 72 and 96 h. After incubation, melanin was extracted and analysed. Our results showed that melanin concentration increased with increasing irradiation dose and exposure duration. Melanin was insoluble in water and organic solvents but soluble in alkaline and acidic solutions. A linear correlation between the logarithm of melanin absorbance and wavelength ranging from 400 to 600 nm was observed. The HPLC chromatograms of melanin degradation products by the alkaline hydrogen peroxide oxidation method revealed the existence of four peaks. The natural melanin standard extracted from the cuttlefish Sepia officinalis was used as a reference compound of eumelanin markers. These markers were detected in control cells, irradiated cells and black human hair but with different intensities.