SUMMARY: Blood plasma was collected under aseptic conditions from cows at various stages of the oestrous cycle; from steers; from oestrous, pseudopregnant and ovariectomized rabbits, and from ovariectomized rabbits treated with either stilboestrol dipropionate or progesterone. The antibacterial activity of plasma samples was tested by three methods using four organisms commonly associated with uterine infection. These methods were: diffusion on seeded agar plates, turbidimetric measurement of bacterial growth, and direct counting. These methods failed to demonstrate any bactericidal factor in blood plasma which can be influenced by ovarian hormones. The results suggest that the mechanisms responsible for the differing bacteridical activity of uteri from animals under the influence of oestrogen and progesterone are not associated with differences in the antibacterial properties of their blood plasma.