Abstract
Immunity to reinfection with the homologous T-mycoplasma strain has been demonstrated in the bovine mammary gland. This immunity was generalized throughout the udder and was not confined to previously infected quarters. Two out of three cows which were immune to reinfection with the homologous strain were not immune to reinfection with a serologically distinct T-mycoplasma strain. Animals varied in their ability to resolve experimental mastitis with T-mycoplasmas: those which resolved the initial infection without antibiotic therapy were immune to challenge with the homologous strain, but those which needed to be given antibiotics to clear the first infection were not all immune to such challenge.