The purpose of the study was to determine the synergistic adjuvant effect of sustained release biodegradable nanoparticles in combination with alum. Nanoparticles containing tetanus toxoid (TT) were formulated using a biodegradable polymer, polylactic polyglycolic acid co-polymer (50:50, molecular weight 100 000). The immunization studies were carried out subcutaneously in rats. The results were expressed as mean serum anti-TT IgG levels. The nanoparticles demonstrated a TT loading of 4% w/w with mean particle diameter of 238 +/- 31 nm. The TT encapsulated in nanoparticles was released slowly under in vitro conditions, with 67.5% cumulative release occurring in 20 days. A single injection of TT-nanoparticles (TT dose = 10 Lf) mixed with TT-Alum (TT dose = 5 Lf) induced a four-fold greater mean serum anti-TT IgG response than a single injection of TT nanoparticles alone (TT dose = 15 Lf) (2235 +/- 310 vs. 539 +/- 49 mug/ml, mean +/- sem, p = > 0.001). In addition, the mean immune response induced with the single injection of combination of nanoparticles and alum was comparable to the two injections of TT-alum alone (5 Lf each dose) given at 3 week intervals IgG = 1998 +/- 333 mug/ ml). Furthermore, the combination induced a peak immune response. IgG = 4215 +/- 546 mug/ml) as early as the first time point at 3 weeks postimmunization. In the case of a TT-alum alone, the animals showed a weaker immune response at 3 weeks and required a second dose of TT-alum to enhance the antibody response. The data thus suggest that the combination of TT-nanoparticles and TT-alum acts as a much better adjuvant than nanoparticles or alum alone. A rapid induction of immune response is useful to curb the spread of communicable diseases in the case of an outbreak.