Recent studies on immunity to helminthic infection have shown that some degree of protective immunity may be stimulated by inoculations of attenuated living worms or their metabolites. The present experiments were designed to observe the effects of attempted immunization in the rhesus monkey by the use of attenuated infective larvae of Brugia malayi.Some effect was observed in animals inoculated with larval incubate, in which microfilaraemia did develop, but at low levels and for short durations. However, the most strking finding was that persistent immunity to challenge infections (expressed as failure to cause microfilaraemia) was obtained in animals vaccinated with large numbers (200) of infective larvae attenuated by X-irradiation at 20 000 R.Experiments with different numbers of larvae attenuated by different doses of irradiation suggest that there is an optimum combined effect of these two factors in eliciting functional antibody in a quantity sufficient to prevent patent infection in Malayan filariasis.