Rats experimentally infected with a California strain of the relapsing fever spirochete, Borrelia hermsi, were examined daily for evidence of spirochetemia and at 5-day intervals for antibody. B. hermsi produced 4 major serotypes (O, A, B and C) in rats. The most significant finding in rats infected Aleutian disease in mink is a transmissible viral disease which occurs in families. In 17 bred females of the as genotype, known to have Aleutian disease, there were 29 dead and 30 live kits in utero. I n 5 bred females of the A a genotype there were 5 dead and 23 live f eti. Of the 5 3 live f eti which were examined 32 contained Aleutian disease virus. Transplacental transmission of Aleutian disease virus does occur.