The leukocyte beta2 integrin CR3 (CD11/CD18), is a surface heterodimeric glycoprotein that functions as a divalent cation-dependent adhesive complex. It mediates several important cell-substrate and cell-cell adhesive interactions among which the interaction with vascular endothelial cells that lead to leukocyte transmigration. We have isolated cDNA clones-coding for the rat complement receptor type 3 (CR3) alphaM subunit (CD11b) from a cDNA library. The cDNA sequence showed respectively 89.4% and 74.6% homology with its mouse and human counterpart. We have expressed the sequence coding for the VA module or Von Willebrand type domain (A-domain) and produced it in E. coli as a soluble recombinant fusion protein with GST. Simultaneously, we have cloned DNA fragments specific to the rat ICAM-1 domain 1 and domain 3 and expressed each clone in E. coli as recombinant soluble (rs) fusion proteins with GST. Recombinant CD11b A-domain was released from the fusion protein by thrombin cut. Purified ICAM-1 fusion peptides and CD11b A-domain were used to develop a direct binding assay that showed a specific binding between the rat ICAM-1 Ig like domain 3 and CD11b A-domain. These data demonstrate that the IgSF modules can be produced as a soluble recombinant fusion protein and used to study direct binding to the VA module displayed by members of the integrin superfamily.