EN
Several components of sheep blood extensively catabolize nucleosides, including thymidine. Part of the radioactivity from tritiated thymidine, in whole blood cultures, is quickly rendered unincorporable as the compound is metabolized to thymine and further breakdown products. Thus, cells continue to proliferate without incorporating radioactivity from the medium. Furthemore, variability in the degree of catabolism varied from animal to animal, so that neither measurement of the depletion of radioactivity from the medium nor measurement of the amount of label incorporated into cultures can be used as a quantitative index of the cell proliferation or immunocompetence.