THE respiration of the dormant spores of aerobic bacteria is always very small, and therefore difficult to measure. Though various authors had given very much larger values for the respiration (Q), H. Halvorson et al.1 have come to the conclusion, based on a consideration of the limitations of the manometric techniques employed, that the true value of Q must be less than 0.05 for spores of Bacillus cereus even in presence of glucose-much less than for vegetative cells. (Q is generally given as µl. oxygen absorbed per mg and h; but for respiration of carbohydrate this is identical with µl. of carbon dioxide produced per mg and h.) An actual value could, however, not be given by Halvorson et al. We have now put to use the extraordinary sensitivity of radiochemical methods to measure both the endogenous and the exogenous respiration. For reasons of economy, the radiocarbon was introduced into the bacteria in the form of photosynthate.