Spores harvested by conventional methods could be cleaned up considerably using discontinuous density gradients. This method yielded a population of almost entirely viable spores. Germination of the spores resulted in density changes probably due to spore swelling. These changes could be observed as early as 15 min after activation. The usefulness of this method in the study of spore activation was shown by its application to spores germinating after a heat treatment at various temperatures, or to spore reacquiring dormancy by incubation in water after activation at different temperatures. Dormant and activated spores can quickly and easily be separated from mixed populations.