In vivo, matrix metalloproteinases are produced in response to ultraviolet B (UV-B) irradiation and are considered to be involved in connective tissue alterations observed in photoaging. The respective roles of keratinocytes and fibroblasts in UV-B-induced MMP-1 production were investigated in monolayer cultures of keratinocytes and fibroblasts as well as in an epidermis model reconstructed in vitro. In contrast to fibroblasts, which secreted MMP-1 in response to UV-B irradiation, no accumulation of MMP-1 was observed after UV-B irradiation of keratinocytes. However, culture medium from UV-B-irradiated keratinocytes, which showed an increase in IL-1alpha and IL-6, induced MMP-1 production by human fibroblasts, suggesting that UV-B irradiation modulates MMP-1 production via both direct and indirect mechanisms.