SUMMARY The envelope proteins of Klebsiella aerogenes (syn. Aerobacter aerogenes) grown in glucose-, sulphate-, phosphate-, ammonia-, potassium- and magnesium-limited environments, in chemostats, have been isolated, and compared by SDS-poly- acrylamide gel electrophoresis ; marked differences were evident. The envelopes from glucose- and sulphate-limited organisms were examined further : protein content was growth-rate dependent, but sulphate-limited envelopes always con- tained less protein than glucose-limited envelopes and this protein had a lower sulphur content. The sulphate-limited envelopes contained one major protein component with a molecular weight of 30000 daltons whereas the glucose-limited envelopes contained three main protein components (molecular weights of 46000, 38000 and 28 500 daltons). Selective extraction of membrane proteins with Triton X-IOO indicated that both wall and membrane proteins altered in response to changes in the growth environment. Similarly, the soluble proteins of the organisms varied, but the ribo- somal proteins remained almost constant.