The relative importance of the environment versus the physiology of the host in the control of Gram negative bacteria on skin has been examined by two experimental designs. (I) The changes in incidence of Gram negative carriage from three skin sites in a climate controlled chamber at 35 degrees C and 90% humidity for 64 h were determined. Following 2 weeks of tetracycline treatment the above experiment was repeated. High temperatures and humidity increased the overall frequency of isolation of Gram negative bacteria although there were individual differences. This effect was reversed 24 h after a return to the natural environment. Tetracycline inhibited the increase in isolation of Gram negative organisms in six subjects, enhanced it in three and had no effect on one subject. Gram negative populations were less than 10 colony forming units/cm2 of skin in most instances but ranged up to 10,000/cm2 skin. No residual effect occurred after return to natural environment and at no time was there a loss of normal flora. (2) Klebsiella pneumoniae, Proteus vulgaris and Pseudomonas aeruginosa were inoculated on normal skin of ten volunteers and survival determined during a 24 h period. Viability was lost between 2 and 6 h on most subjects as compared to 24 h or longer for K. Pneumoniae and P. vulgaris on an inert surface and between 8 and 24 h for Ps. aeruginosa. It was concluded that extremes of environment can affect Gram negative skin carriage but under normal conditions in healthy individuals there is evidence of physiological control of Gram negative flora.