SummaryOBJECTIVE There is increasing evidence that TSH activates a non‐cyclic‐AMP‐dependent pathway in the thyroid resulting in protein kinase C activation. We have previously demonstrated that TSH activates protein kinase C by causing translocation of protein kinase C from an inactive cytosolic site to its active membrane‐bound form in porcine thyroid cells. In addition, TSH can modify the protein kinase C translocation induced by the phorbol ester, 12‐O‐tetradecanoylphorbol‐13‐acetate. We tested six TSH receptor antibodies for their ability to activate protein kinase C invitro.DESIGN Porcine thyroid cells were incubated with either TSH receptor antibody or immunoglobulin from pooled normal sera. Subsequently, cytosol and membrane compartments were separated and protein kinase C assessed in each compartment.PATIENTS The sera utilized in this study were obtained from patients with confirmed Graves' disease as defined by clinical evidence of thyrotoxicosis, increased free thyroxine index and suppressed TSH and increased radioactive iodine uptakes.MEASUREMENTS Protein kinase C was measured in the cytosol and membrane compartments of the porcine thyroid cells using an enzyme assay.RESULTS Compared to immunoglobulin from pooled normal sera, none of the six TSH receptor antibodies affected either cytosolic or membrane‐bound protein kinase C in porcine thyroid cells. The tumour‐promoting phorbol ester, 12‐O‐tetradecanoylphorbol‐13‐acetate, caused protein kinase C translocation. However, the presence of TSH receptor antibody did not modify the phorbol‐induced protein kinase C transiocation.CONCLUSION These results indicate that unlike TSH, TSH receptor antibodies neither cause translocation of protein kinase C nor modify phorbol‐mediated protein kinase C translo