BSA was entrapped into PLGA 50:50 microspheres and the in vitro release study was performed. Then 1 microg of microencapsulated antigen was subcutaneously administered to Balb/c mice and the serum Ig G response was compared to that obtained after the subcutaneous administration of the same amount of only free antigen or of free antigen emulsified 1:1 with Freund's complete adjuvant (FCA). The specific serum Ig G responses obtained from the microencapsulated antigen were higher than those obtained from the free antigen and similar to those obtained from the antigen emulsified with FCA. Therefore, the immune response obtained with the subcutaneous administration of 1 microg of microencapsulated antigen was used as a positive control to compare the immune response elicited by the administration of the spheres either by the oral or the intranasal route. There is a dose/response relationship in the serum Ig G response elicited after three consecutive oral administrations of microencapsulated antigen at a dosing range from 200 to 50 microg. However, this relationship does not seem to be clear for the intranasal administration of the spheres at the same dosing range. When comparing the serum Ig G responses at each dosing level for the different routes of administration, it can be observed that the intranasal route is a more powerful inducer of the production of specific Ig G antibody than the oral, which could be due to a greater permeability of the nasal epithelium and to qualitative differences between the mechanisms of induction of the immune response by each route. The serum Ig G2a versus Ig G1 ratio is not significantly different among all the groups that received antigen in microspheres.