The hypothesis that experimentally determined survival times of Treponema pallidum in stored donor blood could be related to the number of treponemes initially present in the treponeme-blood mixtures was investigated by inoculating rabbits with three graded doses of treponemes suspended in donor blood and stored at 4 degrees C for various periods of time. The storage periods up to which living treponemes could be detected in the testes of the inoculated rabbits as well as those storage periods up to which seroconversion occurred are related to the number of treponemes present in the treponeme-blood mixtures. Increasing numbers of treponemes present in the donor blood resulted in longer periods up to which positive results in both parameters were found. All available evidence suggests that the upper limit of seroconversion coincides with the upper limit of treponemal survival. Inoculation with 5 X 10(4) treponemes per ml of donor blood resulted in a treponemal survival time of 48 h, inoculation with 1.25 X 10(6) treponemes per ml in a treponemal survival time of 72 h and inoculation with 2.5 X 10(7) treponemes per ml donor blood in a survival time of 120 h.