Previous work on the encapsulation of proteins and antigens in poly(lactic-co-glycolic acid) (PLGA) microspheres has led to the conclusion that microencapsulated antigens are frequently inactivated due to their interaction with the polymer. To improve the compatibility of the antigen with the polymer, we have devised a novel microencapsulated system consisting of a blend of PLGA 50:50 and poloxamer 188 (Pluronic F68) and applied it to the delivery of tetanus antigen. Tetanus toxoid was encapsulated in microspheres containing different amounts of poloxamer using an anhydrous procedure based on an oil-in-oil solvent extraction process. The compatibility of the polymers was studied by Fourier transform infrared (FT-IR) spectroscopy. Microspheres were assayed in vitro and in vivo for their ability to deliver active antigen for extended periods of time. Analysis by FT-IR spectroscopy evidenced the miscibility of both polymers by a hydrogen bonding mechanism. In vitro release studies revealed that microspheres containing poloxamer released antigenically active TT, in a pulsatile manner, for up to 50 days. In addition, it was observed that the intensity and duration of the pulses were dependent on both poloxamer content and TT loading in the microspheres. The in vivo evaluation of this new system showed that the neutralizing antibodies elicited by the TT encapsulated in poloxamer-PLGA microspheres were considerably higher and more prolonged than those obtained after administration of the aluminum phosphate-adsorbed toxoid. These results indicate the importance of devising new microencapsulation approaches specially adapted for preserving the activity of protein antigens incorporated within PLGA microspheres.