Given the importance of adequate passive immunity in the neonatal period, the intravenous administration of plasma has been recommended for foals with inadequate colostral immunity. However, the efficacy of plasma treatment has been poorly evaluated. Previous studies on neonatal equine immunity have focussed almost exclusively on the antibody (IgG) status of foals and, to date, little work has been completed evaluating other aspects of neonatal equine immunity, such as leucocyte function. Differences between the response of clinically normal foals with suboptimal passive transfer and those that are septicaemic have not been critically evaluated. Current recommendations are for the prophylactic treatment of all foals with Asuboptimal@ IgG concentrations. However, the necessity for, or benefits of, such treatment has not been evaluated.