The amount of antibody transmitted to the circulation from immune rat serum administered by mouth in a single dose to 12-day-old rats is directly proportional both to the antibody concentration and to the volume of serum administered for doses up to 0.10 ml. The amount of antibody transmitted does not increase with larger doses and is limited to that sufficient to produce a mean circulating titre of 1/32 of that of the immune serum. The uptake of the antibody-containing globulin fraction from a single feed is sufficient to account for the whole of the increase in this fraction of the young rats' serum protein over a period of at least 9 h. The proportion of antibody transmitted to the circulation from immune rabbit serum administered by mouth declines nearly linearly with increasing volume of dose up to the maximum that could be administered of 0.60 ml. The proportion of rabbit antibody transmitted is approximately 1/8 of that of rat antibody from corresponding doses of immune serum up to 0.10 ml. Admixture of certain heterologous sera with the immune rat or rabbit serum administered by mouth interferes with the transmission of antibody to the circulation to an extent that cannot be accounted for by the dilution. Human, ox, guinea-pig and rabbit sera have a marked effect, whereas sheep, mouse, hamster and rat sera have little or no effect. Interference with the transmission of rat antibodies results even when the volume of the dose of mixed sera is less than 0.10 ml. Although interference decreases the proportion of antibody transmitted to the circulation it does not reduce significantly the maximum amount that can be transmitted.