We have previously reported a murine lymphocyte surface antigen MALA-2 of approximately 95,000 Mr which is expressed mainly on activated lymphocytes. The rat monoclonal antibody YN1/1 that detects this antigen profoundly inhibits mixed lymphocyte response. We have now purified MALA-2 and determined its partial amino acid sequence. By using non-redundant synthetic oligonucleotides as probes, based on the amino acid sequence, we have isolated two full length cDNA clones encoding MALA-2. The two clones are identical except for the 5' end sequence. Expression of MALA-2 on transfected COS cells is only achieved with one of the two cDNA clones. The nucleotide sequence as well as the deduced amino acid sequence of MALA-2 display striking homology with those of the recently reported human intercellular adhesion molecule ICAM-1. All the unique features of the human ICAM-1, including its homology with the neural adhesion molecule NCAM, its internal repeat structure and the immunoglobulin-like structure, are found in MALA-2. Furthermore, purified MALA-2 crosslinked to a solid support binds Con A blasts that express LFA-1, the putative receptor for ICAM-1, and the binding can be blocked by YN1/1 antibody or antimurine LFA-1 antibody indicating a direct interaction of these molecules in cell adhesion. Therefore, we consider MALA-2 to be the murine homolog of human ICAM-1. Since ICAM-1 is known to be of primary importance in immune responses and inflammatory reactions, having a monoclonal antibody and a mouse model will provide the opportunity to study the functional role of ICAM-1 in vivo.