Abstract
When fresh animal serum was dropped onto seeded mycoplasma agar plates, inhibition of growth frequently occurred. This effect was dependent on the mycoplasma serotype and on the animal species from which the fresh serum came. This activity of fresh animal serum was heat-labile and would not diffuse through the agar medium. Growth of all the porcine mycoplasma serotypes was inhibited by fresh sheep serum. M. hyorhinis, M. hyopneumoniae, B 3 and the P 45 strains were insensitive to fresh horse serum. The addition of fresh horse serum to specific M. hyorhinis rabbit antiserum-impregnated disks appeared to have a synergistic effect and the combination of M. hyorhinis antiserum-impregnated disk and fresh horse serum always inhibited the growth of M. hyorhinis strains.