To elucidate the binding mode of riboflavin to rat lens protein, we examined the effects of light exposure, different pH (4.0, 7.0 and 9.0) and dialysis on the lenticular riboflavin-binding capacity (LRBC). LRBC increased remarkably by light exposure, irrespective of pH. By dialysis, LRBC of a light-exposed incubation mixture decreased remarkably at pH 9.0, however, at pH 4.0 and 7.0, dialysis did not affect it significantly. Decrease in LRBC in light-exposed dialyzed samples at the higher pH may result from release of dialyzable bound riboflavin from binding protein. These data suggest that the dialyzed bound riboflavin at the higher pH may be responsible for a photosensitization of lens protein.