The production and characterization of two new monoclonal antibodies (MAbs), designated MAR4 and MAR5, raised against the partially purified alpha 5 beta 1 integrin, are described. The reactivity of these 2 MAbs on tumor cell lines indicated that they reacted on all the cells expressing the beta 1 subunit independently of the alpha 5 expression. Both MAbs were found to immunoprecipitate on 3 cell lines, a protein of 120 KD corresponding to the molecular weight be the beta 1 chain, in addition to proteins of other MW corresponding to the alpha subunits differentially expressed by these cells. The cross-competition experiments showed that MAR4 and MAR5 recognize the same epitope. These 2 MAbs seem to be useful reagents for the characterization of the VLA expression in tumor cells.