The Biospecialties Branch, MRMPB, 6570 Aerospace Medical Research Laboratories, initiated a research effort to support and supplement the Air Force Aeronautical Systems Division program concerned with the relationship of microbial contamination to deleterious changes in JP-4 fuel systems. Results of this program have shown the following: the majority of JP-4 fuel systems contain low levels of viable microorganisms; microorganisms are primarily associated with the water phase and rarely with the fuel phase of a JP-4 fuel/water system; JP-fuels exert a toxic effect on viable microbial cells; and microbial corrosion of aluminum alloys 7075 2024, and 7178 can occur in the laboratory but does not appear to be the major cause of corrosion in operational systems. Twenty percent ethylene glycol monomethyl ether (EGME) in a laboratory JP-4 fuel/water-bottom acts as an inhibitor to the majority of viable cells present. As a result of engineering design, it is possible that fuel capacitance probes can cease to operate as a result of microbial growth. Micronic filter materials of phenolformaldehyde impregnated paper can support the growth of microorganisms, but growth even after 43 days is insufficient to cause filter clogging, provided the filter is immersed in fuel. In addition, field surveys of JP-4 fuel handling facilities and aircraft have shown that improved housekeeping and the use of EGME have reduced the quantity of microbial contaminants initially found in the jet systems to an insignificant level.