Phagocytic activity of peripheral leucocytes (PA) was measured in heifers during the luteal phase and oestrus and after administration of various doses of oestradiol, in ovariectomized heifers and cows in the early post-partum period. PA was demonstrated in 22.63 +/- 2.49% and 50.61 +/- 3.76% of phagocytes in the luteal phase and oestrus respectively (P less than 0.01, Fig. 1). The phagocytic index (PI) rose parallelly from 2.26 +/- 0.31 to 6.55 +/- 0.64 particles per cell (Fig. 2). Intramuscular administration of a single dose 3 mg of oestradiol dipropionate resulted in an increase of PA from 28.93 +/- 3.34 to 69.60 +/- 3.32 on post-treatment day 3 (P less than 0.05, Fig. 3). A nonsignificant increase of PA was observed in heifers treated with 10 mg oestradiol. Increases of PA and PI in postparturient cows, treated with various doses of oestradiol, were nonsignificant (Fig. 5, 6) owing to a wide variance of the values obtained, which might be due to individual differences in endocrine and metabolic status or to uterine bacterial contamination. The most marked of haematological changes was the increase of eosinophil counts from 317 to 525.10(6).l-1 in a group of cows treated with 10 mg oestradiol and a decrease of the lymphocyte: neutrophil ratio (Tab. I). Both endogenous and exogenous oestrogens stimulate PA of peripheral leucocytes and 3 mg of oestradiol is a sufficient dose to obtain the effect.