Biodegradable microspheres, prepared from various polymers, and containing a model antigen, BSA, were prepared using a double emulsion solvent evaporation method. The microspheres, consisting of poly(L)lactide, poly(DL)lactide, poly(DL)lactide-co-glycolide, polyhydroxybutyrate of various molecular weights and polyhydroxybutyrate-co-valerate, were characterised for their size distribution, drug loading, release kinetics, surface morphology and hydrophobicity. The influence of these properties on the dynamics of the immune response, following i.m. administration, was studied. The hydrophobic nature of polyhydroxybutyrate microspheres, compared with those formed using polylactides was confirmed and the generation of a significant immune response was delayed using these preparations. Also, the time course of immune responses generated using a range of polyhydroxybutyrate molecular weights was examined.