A knowledge of the growth rates of various organisms at the storage temperature of 4 C in the different suspending media used for red blood cells would aid the extension of the thawed storage time of frozen-reconstituted blood beyond the 24 hours allowed by the Food and Drug Administration. Knowing these rates, a prediction could be made that the growth rate would be sufficiently slow and the unit (sterile or minimally contaminated) could be given safely after a longer storage period. The studies reported show that the pathogenic organisms S. aureus, Ps. aeruginosa, E. coli, Klebsiella sp. and Enterobacter sp. grow at such a slow rate at 4 C that they do not represent any great hazard to the recipient unless introduced in great numbers. The studies further show that in the process of washing frozen blood the number of organisms is reduced by between one and two orders of magnitude (base 10). Therefore, extension of frozen red blood cell storage life to at least 72 hours should be considered.