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The effect of vaccination on rates of microfilarial clearance using Dirofilaria immitis in male Lewis rats was examined. Animals were immunized with whole, dead microfilariae or a PBS extract of microfilariae in Freund's adjuvant. The immunized animals, as well as untreated and adjuvant controls, were challenged intravenously with 4 × 105 viable microfilariae (mf). The duration of microfilaremia was 15.5 days in rats vaccinated with whole mf, 17.7 days in those vaccinated with a PBS extract, 36.3 days for those vaccinated with adjuvant alone, and >70 days for the untreated group. Analysis of the anti-microfilarial IgG response by ELISA and Western blots demonstrated that immunization induced significant amounts of antibody against high molecular weight peptides, particularly a peptide located at 105 kDa. Antibody levels in both groups of immunized animals continued to rise following challenge, reaching peak levels of 78–80 µg/ml on the day of microfilarial clearance. Decreasing microfilaremia following challenge was associated with an enhanced recognition of low molecular weight peptides.