Screening of a rabbit breeding colony by the India-ink immunoreaction test revealed that 73% of the animals had antibodies to Encephalitozoon cuniculi. The study also indicated the passive transmission of antibodies from infected dams to their offspring. Ninety-five percent of the rabbits born to seropositive dams had maternal antibodies to Encephalitozoon cuniculi up to the age of 4 weeks with titers ranging from 1:25 to 1:800. After elimination of maternally transferred antibodies, about 50% of the young rabbits seroconverted; and from the age of 8--10 weeks, serum antibody titers over 1:800 were detectable. When these animals were tested again at 14 weeks, most of them had titers of 1:1600 or higher.