Experimental conditions were adjusted so that they could show the effect of opsonization of bacteria with complement, represented here by fresh murine serum, which contributes significantly to infection of B cells by F. tularensis. Unexpectedly, it was clearly demonstrated that filipin and cyclodextrin inhibited the entry of F. tularensis into peritoneal CD19+ cells only under opsonic conditions. This evidence concerns the gene CD19 and infection.