The susceptibility of early-passage feline brain cells and Crandell feline kidney (CRFK) cells to infection with three isolates of feline immunodeficiency virus (FIV) was investigated. The Petaluma strain of FIV could well infect both the feline brain cells and CRFK cells. The KYO-1 strain could well infect the feline brain cells but the replication in CRFK cells was demonstrated only by coculturing fresh feline T-lymphoblastoid cells with the infected cells. On the other hand, the TM1 strain could infect the feline brain cells but not CRFK cells. Moreover, the replicative ability of the TM1 strain in the feline brain cells was much less than the KYO-1 and Petaluma strains. These results indicate that biological differences can be detected among the FIV isolates.