This study aimed to assess the potential merits of formulating sustained-release microspheres of recombinant human granulocyte macrophage colony stimulating factor (rhGM-CSF) via freezing-induced phase separation (FIPS) of the protein with dextran followed by encapsulation with binary mixture of poly(lactic-co-glycolic acid) (PLGA) 2A (MW similar to 12K) and 3A (MW similar to 47K) or of PLGA2A and polylactic acid (PLA; MW similar to 83K). The formulated dextran particles and microspheres were characterized in vitro for loading, aggregation, bioactivity and release behavior of the protein where appropriate. rhGM-CSF retained about 60% of bioactivity with no significant aggregation after each formulation step. Encapsulation of protein-loaded dextran particles attained only 80% with the PLGA2A and PLGA3A blend, but 100% with the PLGA2A and PLA mixture. The former formulation exhibited a triphasic in-vitro release profile typical of PLGA microspheres while the latter revealed a much lower initial burst followed by a steady and complete release of rhGM-CSF with preserved bioactivity over a 15-day period.