Electron microscopic observations of acutely infected rabbit testes showed that the majority of Treponema pallidum were extracellular, and confined to interstitial regions of the tissue. Organisms were often adjacent to small blood vessels, where they should be freely accessible to non-specific humoral and cellular defence mechanisms. However, there was no accumulation of leucocytes in blood vessels or infiltration of inflammatory cells into infected areas. Inoculation of live treponemes into perforated plastic chambers which had been implanted subcutaneously in rabbits or guinea-pigs did not incite significant infiltration into the chamber fluid of inflammatory cells, in contrast to that seen after inoculation of Neisseria gonorrhoeae. Interaction of antibody from infected rabbits with live treponemes freshly extracted from rabbit testes could not be detected by an indirect fluorescent antibody method. These observations suggest that T., pallidum escapes recognition by host defences.