“…Measurement of the association of bacteria with animal cells in vitro has most commonly been made by microscopy of stained monolayers (Hartley et al, 1978;Hackney et al, 1980;Tavendale et al, 1983). Some procedures have divided the bacterial-animal cell interaction into two phases, the infection phase and the intracellular growth phase (Lee et al, 1977;Une, 1977 ;Une et al, 1977 ;Pedersen et al, 1979 ;Okamoto et al, 1980;Soemitro et al, 1981), a consideration that appears unnecessary with Y. enterocolitica because it does not multiply intracellularly (Devenish and Schiemann,198 1). Bacteria that do not associate with the animal cells during the interaction period are usually removed by repeated washing (Maki et al, 1978;Portnoy et al, 1981;Mintz et al, 1983) but density-gradient Received 10 Jul.…”