In the present study, the effects of antibodies against specific beta 1 integrin heterodimers on mouse embryo attachment and spreading were tested to identify the role of the beta 1 integrins in early implantation. We developed assays for the attachment of mouse embryos and for trophoblastic spreading on cultured human decidual cells. Blastocysts became attached to the cultured decidual cells in the presence of a purified mouse monoclonal IgG1 antibody (negative control) after the embryos hatched from the zona pellucida. The majority of hatched blastocysts attached within 24 h of culture. Blastocysts that were attached to decidual cells exhibited extensive outgrowth after 48 h. The addition of antibodies directed against the beta 1 and alpha integrin subunits to the cultured decidual cells did not affect the rates of hatching or attachment of the blastocysts. However, the outgrowth of embryos on the decidual cells was inhibited by the addition of a monoclonal antibody against the beta 1 subunit in a dose-dependent manner, implying that blastocyst attachment and outgrowth are mediated by different mechanisms. Although the area of trophoblast outgrowth in the presence of a mouse monoclonal lgG1 antibody was increased during 96 h of culture, anti-beta 1 antibody blocked the outgrowth of trophoblasts in a dose-dependent manner. Both the incidence and extent of trophoblastic outgrowth were also significantly reduced in the presence of antibodies against the alpha 1, alpha 2, alpha 5, and alpha 6 subunits. These observations suggest that beta 1 integrins on decidual cells may be involved in blastocyst development and differentiation following attachment.