SummaryAntigens were prepared from human umbilical cord hyaluronic acid, streptococcal hyaluronic acid, group A hemolytic streptococci and Staphylococcus aureus and these same bacteria incubated with human umbilical cord hyaluronic acid. Rabbits were injected intravenously, intramuscularly and subcutaneously with these antigens. Neither precipitating nor non-precipitating, skin-sensitizing antibodies against hyaluronic acid developed. Anti C carbohydrate and anti M protein antibodies were produced in the animals which received streptococci, indicating that the rabbits had received enough of at least 2 antigens, one a carbohydrate the other a protein, to stimulate antibody production. The development of skin sensitivity to hyaluronic acid was suggested but by no means proved. These experiments furnish further evidence that hyaluronic acid is probably not antigenic by itself, nor does it act as a hapten capable of adsorption to group A hemolytic streptococci or Staphylococcus aureus.