Lipid vehicles and surfactive agents have been successfully used to increase oral absorption and availability of free and encapsulated proteins. In order to investigate if these vehicles could also enhance the serum IgG responses elicited after the oral administration of protein antigens, free bovine serum albumin ( BSA) was orally administered to Balb/c mice in different vehicles: a 0.3% sodium bicarbonate aqueous solution, an ethyl oleate/ 0.3% sodium bicarbonate o/w emulsion ( 1: 9 v/v containing 0.01 m sodium deoxycholate and 1% poloxamer 188) or ethyl oleate containing the previously described surfactive agents. The immune response elicited by the free antigen was enhanced by the use of these substances, especially when the free protein was administered as an oil suspension containing the surfactive agents. However, when protein loaded 1 mm PLGA particles were orally administered, the use of these enhancers did not result in an improvement of the serum IgG responses, and only the suspension of the spheres in ethyl oleate containing the poloxamer and the bile salt elicited a similar immune response to that achieved with their suspension into an aqueous solution without any enhancer, which suggests that these enhancers are not capable of increasing the absorption of particulated antigens.