The effects of variations in temperature within the physiologic range on minimal inhibitory concentrations (MICs) and on the serum bactericidal activity of 17 different antimicrobial agents for 432 strains of bacteria were studied. Comparison of 3,053 experimental MICs performed at 41.5, 40, 38.5, 37, or 35 C with duplicate standard MICs performed at 35 C showed a progressive increase in antimicrobial activity as the temperature was raised within the experimental range. At the highest temperature (41.5 C), 17.1% of MICs were four or more times lower, 7% were eight or more times lower, and 2% were greater than or equal to 16 times lower than the standard MICs performed at 35 C. Binding to proteins in serum neither accentuated nor diminished the augmenting effect of temperature on antimicrobial activity. Comparison of serum bactericidal activity determined at 35 C and 40 C revealed similar hyperthermic augmentation of antimicrobial activity.