The influence of storage at 4°C on the survival of Treponema pallidum in donor blood, artificially infected with treponemes of the virulent Nichols strain was studied. Rabbits were inoculated in each testis with 0.5 ml of the blood-treponeme mixture, containing 5 X 10^5 microorganisms/ml, which was stored for up to 336 h. Under these circumstances, the blood-treponeme mixture appeared to be infectious for up to 96 h of storage, based on the demonstration of T. pallidum and serological findings, using FTA-ABS, TPHA and YDRL tests. The two former tests were found to be equally sensitive in detecting incubating syphilis in the rabbits, the VDRL test was significantly slower in this respect. From the absence of orchitis as well as seroconversion in rabbits, inoculated with bloodtreponeme mixtures stored for 120 h or longer periods of time, it is concluded that after 120 h of storage infectivity is lost. The presently found survival time of treponemes in blood between 96 and 120 h of storage is discussed in relation with survival times found by other investigators.