It has been pointed out in rheumatic fever that the extracellular products of streptococcus (EPS) induce an immune response capable of producing autoimmunity against cardiac structures. In an experimental model in rats, the possibility that EPS facilitates the production of antisarcolemma antibodies was analyzed by injecting the animals with EPS, or simultaneously with EPS and human heart sarcolemma (HHS). The rat sera was analyzed by microdouble immunodiffusion against EPS, HHS or a mixture of both, the rats immunized with HHS or with EPS and HHS produced anti-HHS antibodies. The average of precipitation bands increased with the number of immunizations. Under the experimental conditions used, the humoral immune response against HHS was not modified by EPS and also EPS alone did not induce antibodies against sarcolemma. The synthesis of antibodies to EPS was very low or Null. The antiestreptolysin-O levels were low during the immunization period. A minimal inspecific myocarditis with lymphocytic inflammatory infiltrate was induced. The humoral immune response against cardiac sarcolemma and against the extracellular products of streptococcus was similar in the groups of rats studied, independently of sex, weight, age, dose, inoculation method or immunization time in accordance with schemes used in this work.