The monoclonal antibodies HML-1, B-ly7 and Ber-ACT8 recognize intramucosal gut T lymphocytes, activated cells, and hairy cell leukemia. The antigen on hairy cells consists of three glycoproteins (160kD, 130kD and 105kD unreduced; 145kD and 120kD reduced). These peptides have biochemical features reminiscent of integrins but we have shown by immunoprecipitation that they are not known integrin subunits. We have used a newly produced antibody (BP6) to purify this molecule and shown by N-terminal sequence analysis that the smallest subunit is the product of integrin beta-7 cDNA. This molecule is thus a new member of the integrin family of leucoyte adhesion proteins. Immunoprecipitation experiments indicate that the two larger subunits are recognized by HML-1, B-ly7 and Ber-ACT8.