Anti HLA Class I Monoclonal Antibody depletes Protein Kinase C (PKC) to 20% of control value in PHA activated human T cells. The effect is reversible: in 24 hours the enzymatic activity returns to 58% of control value. Removal of antibody from the culture medium increases the rate of recovery. Implications of this finding for the modulation by HLA Class I antigens of the proliferative response of T cells to lectins are discussed.