Leaf expansion, comprising cell division and cell enlargement, is controlled by light quality and quantity. The role of UV-B irradiance on leaf cell enlargement has not been determined. We studied the effect of a wide range of UV-B irradiances on the cell-enlargement-driven expansion of Phaseolus vulgaris L., cv. Contender (bush bean) leaf discs. Our growth method allowed separation of the cell enlargement phase of leaf expansion from the cell division phase. In two series of experiments with different types of UV-B screening filters, the effect of increasing levels of UV-B on the area of excised P. vulgaris leaf discs was investigated. One set of experiments utilized polyester (UV-B-absorbing) and cellulose acetate (UV-B-transmitting) filters. The other set utilized UV-B-absorbing and UV-B-transmitting acrylic filters. Regardless of which type of filter was used for screening, high (above summer solstice) levels of supplemental UV-B inhibited cell enlargement in a linear, dose-dependent manner, resulting in smaller leaf discs than treatment with UV-B-absorbing filters. Conversely, low levels of supplemental UV-B enhanced cell enlargement in a linear, dose-dependent manner, resulting in larger leaf discs than did treatment with UV-B-absorbing filters. The results suggest a biphasic response to UV-B, and that there is an optimum UV-B level that results in maximum leaf expansion by cell enlargement.