To analyse the mechanisms of resistance against infective larvae of Brugia malay induced by 3 times' vaccination with 100 irradiated larvae, passive transfer of protective immunity by serum and/or spleen cells from vaccinated BALB/c mice to normal mice was examined. Resistance was observed by the worm recovery in the peritoneal cavity of mice 2 weeks after intraperitoneal challenge infection with 100 infective larvae. No larva was recovered in the recipients of 1.5 X 10(8) spleen cells from vaccinated mice, whereas an average 25.89% of larvae were found alive in control mice transferred with the same number of spleen cells from age-matched non-vaccinated donors. However, passive transfer of either 0.1 or 1.0 ml immune serum failed to protect the recipient mice against the challenge infection. Furthermore, increased larval recovery was induced by challenge infection with larvae which were previously incubated in 10% immune serum. The results suggest that an antibody-dependent immunological enhancement may occur in these experiments.