ABSTRACT Sera from three pigs injected subcutaneously with respectively pig, ox, and horse insulin for 87 days were incubated with 131I-pig insulin and investigated for the presence of antibodies by means of immunoelectrophoresis and autoradiography. A binding of 131I-pig insulin to the gamma globulin fraction could be demonstrated in all animals after the commencement of the injections. Thus a cross reactivity between pig and ox insulin and pig and horse insulin occurred, however, apparently to a lesser degree in the latter case. The presence of a 131I-pig insulin binding gamma fraction in the serum from the animal injected with pig insulin, indicates a formation of antibodies against pig insulin in the pig.