Abnormal accumulations of prion protein (PrP) can be detected in the spleen, lymph nodes, and tonsils of patients with variant Creutzfeldt-Jakob disease (vCJD). Therefore, it has been assumed, but not shown, that these tissues harbour infectivity, which in turn presents the potential for iatrogenic spread through surgery. Here, we show and measure levels of infectivity in spleen and tonsil from two patients with vCJD, by bioassay in intracerebrally inoculated RIII mice. Similar bioassays failed to detect infectivity in buffy coat and plasma.