Guinea-pigs given a single injection of bacteriophage ΦX174 produced 19S antibody 8 days later. The levels of 19S antibody attained were not significantly different in animals injected with bacteriophage in water-in-oil emulsions with and without added mycobacteria, in bentonite or in saline alone. On the other hand, while both 7S γ1- and γ2-immunoglobulin antibodies were present 21 days after a single injection of bacteriophage ΦX174 in saline alone, the addition of water-in-oil emulsions to the injection mixture led to enhanced levels of 7S antibody and the addition of mycobacteria to these emulsions increased 7S antibody levels still further. Delayed hypersensitivity reactions were detected by corneal testing in a proportion of the guinea-pigs injected with bacteriophage in water-in-oil emulsions whether or not mycobacteria were also present in the mixture. Guinea-pigs immunized with bacteriophage in saline showed negative corneal tests.