Melanophores are a class of chromatophore found in the dermal layer of amphibian skin, which contribute to rapid and reversible changes in skin colouration. These cells contain thousands of melanosomes, a lysosome-related organelle, able to synthesise and store the brown-black pigment melanin. Melanosomes may be dispersed throughout the cell, or may become aggregated in the perinuclear region. The distribution of melanosomes within melanophores can be dramatically altered by various hormones, neurotransmitters and paracrine factors. Light can also trigger melanosome dispersion (Daniolos et al. 1990). This study examined the characteristics of light-induced melanosome dispersion in a clonal line of X. laevis melanophores. Melanophores (~4,000 cells/well) were seeded in black-walled, clear-bottomed 96-well plates 48 hr before use in 0.7x Leibovitz L-15/15% FCS medium. Pigment granule translocation was monitored by measuring the change in cell absorbance at 630 nm. Photodispersion on exposure to white light (100 μW/cm²) could be detected after 5 min, reached a maximum after ~60 min, and was evident both in untreated, dark-adapted cells and in melanophores pre-aggregated with melatonin (100 pM, 1hr) in darkness. Measurement of the change in pigmented area using ImageJ (http://rsb.info.nih.gov/ij/) in individual melatonin-treated cells after light exposure revealed that ~95% of cells (67/70) showed significant photodispersion (>10% increase in pigmented area) with a mean increase in pigmented area of 117±9%. Dispersion increased with increasing light intensity and an irradiance-response curve to white light was generated and gave a mean IR₅₀ (irradiance giving 50% dispersion) of 56 ± 1 μW/cm² (n=8). The spectral sensitivity of photodispersion was determined by using interference filters to generate irradiance-response curves at 7 wavelengths (456-578 nm). The IR₅₀ at each wavelength was determined from which the relative sensitivity at each wavelength of light was calculated and an action spectrum (log relative sensitivity vs. wavelength) was constructed. The data were well-described (r² = 0.8) by the characteristic absorption spectrum of a vitamin A-based opsin photopigment with a wavelength of maximal sensitivity (λ_max) of 500 nm indicating that photodispersion is mediated by an opsin-like photopigment. A comparison of the action spectrum with the standard absorbance templates for the known X. laevis violet cone, green rod, red rod and red cone opsins demonstrates that these classical image-forming opsins do not mediate photodispersion, and suggest that a novel photoreceptor is responsible (Berson, 2003).