Endothelial cells express a 400‐kDa or 240‐kDa laminin α chain, depending on their tissue of origin or physiological state [1, 2]. Using differential display and subsequent screening of a mouse endothelial cell cDNA library we here identify the gene coding for the 240‐kDa laminin chain as the laminin α4 gene. The complete mouse laminin α4 cDNA sequence is reported and compared with other laminin α chains. In situ hybridization of embryonic and new born mouse tissues revealed expression of laminin α4 mRNA in a subset of endothelium, in particular aortic endothelium, endocardium and endothelium of blood vessels in the skin and in the brain. Strong laminin α4 expression by aortic endothelia was confirmed by data obtained from cultured bovine aortic endothelial cells (BAEC). Isolation of laminin from BAEC conditioned medium revealed a Y‐shaped molecule in rotary shadowing. Subsequent sequencing of BAEC laminin resulted in laminin α4, β1 and γl amino acid sequences, confirming that laminin α4 is one of the major laminin α chains expressed by aortic endothelium not only in the mouse. In addition, strong laminin α4 mRNA expression occurred in peripheral nerves, cardiac muscle, fat, the dermis of the skin and lung stroma of mouse tissues. The data demonstrate a cytokine and progesterone‐regulated differential expression of laminin α4 mRNA in mouse endothelium, suggesting a distinct functional role for this laminin chain in endothelium.