The virulence of 74 isolates from clinical cases and food products and of 11 isolates of other species was tested in mice immunocompromised with carrageenan. Isolates of species other than were not lethal to such mice. All 29 clinical isolates of (serotypes 1/2a, 1/2b, 4b) and 33 of 42 isolates of various serotypes isolated mainly from dairy products killed all test mice (100% lethality) at an inoculum of 10 cfu/mouse. All lethal strains of were haemolytic and possessed the 58-Kda band specific for listeriolysin O as demonstrated by SDS-PAGE immunoblotting. The nine avirulent strains of had detectable haemolytic activity, but in six of them this activity was significantly weaker than in virulent strains and the 58-Kda band was not detected. The other three avirulent strains were highly haemolytic and possessed the 58-Kda band, which suggests that other factor(s) could be involved in the virulence of .


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