We examined the relationship between blocking type anti-TSH receptor antibody (TRAb) and stimulating type TRAb by trying to convert blocking type TRAb to the stimulatory type in vitro. Immunoglobulins (IgGs) purified from sera of six patients with primary hypothyroidism blocked bovine TSH (100 microU/ml)-induced cAMP production (58.3-82.1% inhibition) in cultured porcine thyroid cells. None of these IgGs showed stimulating activity even after their dilution. In the conversion experiment, thyroid cells were first incubated with these IgGs at 34 degrees C for 30 min and then washed with incubation buffer. They were then incubated with various kinds of anti-human IgG antibodies first for 1.5 h at 4 degrees C and then for 18 h at 34 degrees C, and the cAMP concentrations in the supernatants were measured. All six IgGs showed strong, dose-dependent thyroid-stimulating activity after addition of antibodies against human whole IgG, or Fab or Fc fragments of IgG. The Fab or F(ab')2 fragments of goat anti-human IgG antibody also had these converting activities, although less than whole IgG. Addition of normal IgG in the first incubation or anti-human IgG antibody alone had no thyroid-stimulating activity. Anti-human IgA or IgM antibodies did not have these converting activities. These results show that blocking type anti-TSH receptor antibodies can be converted to the stimulating type by anti-human IgG antibodies in vitro. The results suggest that the blocking and stimulating types bind to the same epitope(s) of TSH-receptor related antigens. The same anti-TSH receptor antibody may act as a stimulator or blocker by the influence of other factors, such as anti-idiotype antibody.