The ability to infect lymphoreticular organs is a long recognised feature of TSE agents, and there is evidence that extraneural replication can play a pivotal role in the pathogenesis of the infection. We have studied the lymphotropism of sheep scrapie, BSE and CJD infectious sources in transgenic mice expressing either ovineVRQ (tg338) or humanM129 prion protein (tg650) by examining the accumulation of PrPSc and infectivity in their spleen. It was found that tg338 mice intracerebrally inoculated with a class of scrapie primary isolates common in the field accumulated prions with clearly distinct strain properties in the spleen and brain tissues, based on their PrPres molecular profile and biological properties on further passaging. Such a dual distribution was altered upon inoculation by peripheral route. Biological cloning allowed the individualisation of two strain components with fairly different tropisms. One of which, designated Lan19K, appeared to be essentially devoid of lymphotropism, similar to that found for Nor98 atypical scrapie propagated on the same mice. In contrast, another group of scrapie isolates promoted PrPres accumulation at equivalent levels in the spleen and in the brain. These data indicate that the lymphoinvasive potential of natural scrapie strains is quite variable, and that this criterion could be useful as an aid to further investigate their diversity in mouse bioassay. Similar strain-related discrepancies were observed following transmission of classical and atypical BSE agents. Transmission of human TSE isolates to tg650 mice revealed a prominent infection of lymphoid tissue by vCJD agent while only minimal involvement was observed in the case of sCJD agent, thus faithfully reproducing the situation reported in infected humans. Intracerebral inoculation of a reference vCJD sample led to the propagation of prions with either vCJD or sCJD strain-like properties in the brain, but invariably vCJD-like in the spleen. Remarkably, following inoculation by peripheral route, vCJD agent failed to produce an efficient neuroinvasion leading to a clinically silent, sustained extraneural infection along the whole mouse life. Collectively these observations show that prions with different strain properties can propagate concomitantly in the nervous and lymphoid tissues, and point to further complexity in the interactions that natural prion agents can establish with the infected host.