Abstract: The ability of Quillaja saponaria extract to enhance passive immunization in a pig model was investigated. On day (D) 2 of age, pigs were assigned to one of two treatments receiving extracts of Saponin (Quillaja saponaria), which were administered orally on D3 and D4 of age (5 mg/kg body weight in saline; n = 24), or an equivalent volume of saline (Control; n = 25). Piglets were weighed and serum immunoglobulin (IgA, IgG and IgM) concentrations measured on D4, D8, D16 and D32 of age. In initial analyses, a strong relationship was found between neonatal pig weight (D2 of age) and the majority of measured parameters. Hence, within treatment, pigs were blocked by body weight, High (> 1.50 kg), Moderate (1.0 to 1.5 kg), or Low weight (< 1.0 kg). Saponin treatment did not affect serum immunoglobulin concentration of piglets from the High or Moderate weight groups; however, D4 serum concentrations of IgA tended (P = 0.06) to be higher in the Low weight saponin-treated group compared to controls. Additionally, saponin tended (P < 0.1) to increase the rate (139 ? 6 vs. 111 ? 14 g/day for Saponin vs. Control pigs) and the extent (5.1 ? 0.2 vs. 4.2 ? 0.4 kg, Saponin vs. Control at D32 of age) of growth in the Low weight group. For lower birth weight pigs where the intake of colostrum/milk immunoglobulins is limiting, these data suggest that oral administration of saponin could be useful in enhancing passive immunization, and subsequent health and growth.