It has been suggested that changes take place in human platelets when they are exposed to fluorescent light of 10.8W/m2/sec at 460 nm. Loss of ability to aggregate and increase in turnover rate have been reported. However, studies of the effect of fluorescent light on adult platelets when exposed to the level of irradiance used clinically (3.5W/m2/sec) for 60 min did not reveal this effect. A study was therefore designed to compare the effects of bilirubin and phototherapy alone and in combination on cord and adult blood platelets obtained from jaundiced neonates. Using beta-thromboglobulin as the indicator of platelet damage, no significant change was found in platelet poor plasma from cord or adult blood following exposure to bilirubin and/or light, nor was any change found in the plasma of jaundiced neonates before and after therapy. Although the duration of exposure to light in this study was short, the results suggest that the lower levels of irradiance used clinically are not harmful to platelets and that the results of studies done with three times as much irradiance have little clinical relevance.