The mAb A6 was produced by immunization of mice with human PHA-stimulated PBMC. Immunoprecipitation studies and staining of cell lines transfected with individual leukocyte common antigen (LCA) isoforms showed that A6 recognizes a unique epitope strongly expressed on the lower MW isoform (p180) of LCA, but also weakly expressed on the p190 isoform coded by exon B and the p205 coded by exons A and B. The epitope recognized by A6 was carbohydrate-dependent in that it was neuraminidase-sensitive, but trypsin-resistant. A6 strained most TCR-alpha beta+ cells with differential intensities, subdividing them into a bright and dim population, and strongly stained all TCR-gamma delta+ cells. A6 did not stain CD19+ B cells nor CD56+ NK cells. Anti-CD45 mAb such as UCHL1 recognizing CD45RO have been used to define memory T cells. Depletion of PBMC subsets with A6 or UCHL1 mAb dramatically decreased proliferative responses to the recall antigens anti-CD3 mAb and alloantigen and enhanced their responses to PHA. A6, unlike UCHL1, also depleted alloreactive T cells that affect primary and secondary MLC and CML. Thus, A6 was shown to recognize the lower MW isoforms of LCA which are expressed on functional T cell subsets including memory, activated, and alloreactive T cells.