A rat mAb termed 1B11 recognizes a 130-kDa cell surface glycoprotein expressed on T lymphocytes. Transfection studies using the Cd43 gene transfected into murine L cells, and immunoblots using anti-peptide Abs specific for the CD43 polypeptide identified the 1B11 Ag as the 130-kDa isoform of murine CD43. mAb 1B11 fails to recognize the other major CD43 isoform, 115-kDa CD43, either by Western blotting or by FACS analysis, thus differing from the previously characterized anti-CD43 mAb S7 that recognizes only the CD43 115-kDa isoform and not the CD43 130-kDa isoform. CD43 130-kDa recognized by mAb 1B11 is differentially expressed on T lymphocytes. Whereas most CD4-8-, CD4+8+, and CD4-8+ thymocytes express 130-kDa CD43 constitutively, the Ag is expressed by less than 20% of CD4+ T cells in immature and mature populations. On activation, expression of 130-kDa CD43 is up-regulated dramatically on CD4+ T lymphocytes, and to a lesser extent on CD8+ T lymphocytes. In contrast, T cell activation resulted in only minor up-regulation of 115-kDa CD43. CD43 130-kDa contains sialylated O-linked carbohydrate; however, recognition by mAb 1B11 is not dependent on the presence of sialic acid. Interestingly, removal of sialic acid by neuraminidase treatment of 1B11-negative CD4+ T lymphocytes or 1B11-negative EL4 cells confers 1B11 reactivity, suggesting that the 1B11 epitope is masked by sialic acid residues on the CD43 115-kDa isoform. The isoelectric point (pl) of 130-kDa CD43 was determined to be 6.0, which is higher than the pl reported for 115-kDa CD43. Different molecular properties of 115-kDa and 130-kDa CD43 and their differential expression in T cell subsets may indicate specific roles for these CD43 isoforms in T cell ontogeny and/or T cell function.