Subcutaneous injection of beta 1 integrin-deficient embryonic stem cells in mice causes the formation of teratomas although they occur with a lower frequency and are smaller than wild-type cells. Immunofluorescence analysis of these deficient tumors indicates a disorganized deposition of several basement membrane proteins. This was confirmed by electron microscopy which demonstrated frequent gaps in cell-associated basement membranes or loss of close contacts to the cells. Further aberrant features were multilaminar structures and amorphous deposits, indicating a strong impairment of correct basement membrane assembly. Quantitative radioimmunoassays were used to determine the levels of specific proteins in successive tissue extracts with neutral buffer in the absence and presence of EDTA and with 6 M guanidine. This demonstrated a more than 90% decrease in the content of laminin-1 (alpha 1 beta 1 gamma 1) and a 70% decrease in nidogen in the beta 1 integrin-deficient teratomas. No significant changes were detected for other matrix proteins (perlecan, fibronectin, fibulins). This selective change impaired the formation of laminin-nidogen complex and enhanced nidogen degradation. Northern blots also demonstrated a distinctly reduced expression of laminin alpha 1, beta 1, and gamma 1 chains. Similar reductions were also observed in cultured embryonic stem cells prior to any differentiation. No or only smaller changes were observed for laminin alpha 2 and beta 2 chain, nidogen, and perlecan mRNA. These data emphasize a distinct role of beta 1 integrins in the correct assembly of basement membranes which may occur through direct ligand binding and/or regulatory events at the transcriptional level.