Ribosomes from the psychrotroph,Bacillus insolitus, were successfully dissociated into 30S and 50S ribosomal subunits, which were active in carrying out in vitro protein synthesis, measured by the poly U-directed incorporation of14C-l-phenylalanine into polyphenylalanine. As opposed to the undissociated ribosomes, which are heat sensitive (30°C), the dissociated ribosomes were not thermally sensitive. The heat-sensitive component(s) was found to be removed from the ribosomes during dissociation into 30S and 50S ribosomal subunits; when added back to the ribosomal subunits, heat sensitivity was conferred.