The first acceleration of protein degradation in cells of Bacillus megaterium was found at the stage 0-I of sporulation, the second one at the stage II-III, where the sporulation process became irreversible. These accelerations were reduced by actinomycin D inhibiting RNA and protein syntheses by more than 95%. In the presence of the antibiotic, only 8% of prelabeled proteins were degraded. Actinomycin D did not lower either the concentration of ATP or the proteolytic activity in the homogenate prepared from sporulating cells. This indicates that the inhibition of protein catabolism by actinomycin D was not owing to the absence of ATP or proteolytic enzymes. Actinomycin probably inhibited an unknown step preceding the proteolytic attack of the protein molecules during sporulation, because it had no significant effect on proteolysis during vegetative growth.