The distribution of the alpha 1-alpha 6 as well as alpha v, beta 1, beta 3 and beta 4 integrin subunits in human first and second trimester and term placentas was studied by indirect immunofluorescence microscopy using a panel of monoclonal antibodies (mAbs). In first and second trimester villi, the alpha 1 and beta 1 integrin subunits were detected in the stromal cells, that were mostly also immunoreactive for desmin. Desmin-positive stromal cells were also found in villi of term placentas, but the stroma was negative for anti-alpha 1 and -beta 1. In the villous trophoblast, anti-alpha 6 and -beta 4 revealed a distinct basal immunoreactivity during all stages of development, whereas immunoreactivity for the alpha 3 and beta 1 subunits emerged during the second and third trimesters. Throughout placental development, endothelia of villous capillaries reacted prominently with anti-alpha 1 and -beta 1. Intermediate trophoblastic cells displayed a somewhat heterogenous immunoreactivity for the beta 1, alpha 1, alpha 3 and alpha 5 integrin subunits, and differed from villous trophoblast also in their lack of expression of the alpha 6 and beta 4 subunits. While nondecidualized endometrial cells displayed weak reactivity for the alpha 1 and beta 1 integrin subunits, the individual decidual cells presented both a basement membrane and a cell surface-confined immunoreactivity for anti-alpha 1, -alpha 3, and -beta 1. The results suggest a role for integrins in placental development, and show that expression of integrins is modulated during the differentiation of trophoblast, villous stroma, and decidual cells. Furthermore, the basal localization of alpha 6 beta 4 and alpha 3 beta 1 integrins suggests that they are employed as basement membrane receptors in the villous trophoblast, and the emergence of the alpha 3 beta 1 complex may reflect that the cytotrophoblast and syncytiotrophoblast recognize the basement membrane differently.