The molecular nature of the structural changes on the T cell-CD6 glycoprotein upon cell activation has been investigated. Cell surface 125I labeling and immunoprecipitation studies from PBMC revealed that after stimulation by different activators of protein kinase C, or after exposure to either human or FCS, the anti-CD6 mAb precipitated an additional protein of 130 kDa, together with the 105-kDa protein present in resting cells. Cell surface expression of this 130-kDa CD6 protein form could be detected as early as 15 min after PKC activation, without requiring de novo protein synthesis. Pulse and chase activation experiments of radioiodinated cells suggested that the 130-kDa molecule is the result of a posttranslational modification of the 105-kDa protein and that this conversion is a reversible process. Studies of 32P-cell labeling and immunoprecipitation by anti-CD6 mAb revealed that only the 130-kDa form was phosphorylated, whereas the 105-kDa protein was unphosphorylated both in resting and activated cells. Moreover, the removal of phosphate groups from the 130-kDa CD6-form by enzymatic treatment with alkaline phosphatase resulted in its conversion to the 105-kDa form. Taken together, these results demonstrate the existence of two CD6 molecular forms that are in a dynamic equilibrium and differ only at their degree of phosphorylation: a 105-kDa unphosphorylated form present in resting T cells that changes very rapidly to a 130-kDa phosphorylated form by exposure of cells either to serum or to activators of PKC.