Heating of partially purified Staphylococcus aureus MF-31 ribosomes at temperatures ranging from 37 C to 55 C in buffer containing magnesium, phosphate, or ethylenediaminetetraacetic acid (EDTA) caused the degradation of the ribosomes. The degradation in phosphate was similar to that in EDTA but was greater than that in magnesium. Polynucleotide phosphorylase and ribonuclease were implicated as the enzymes involved. Degradation was stimulated by heat at temperatures comparable to those at which sublethal injury and ribosome loss occurred in the intact resting cells.