The toxicity of the fatty acids was much reduced or abolished by (a) esterification, even when the resulting product was a water-soluble ester, and (b) addition of crystalline serum albumin to the culture medium; other proteins tested were inactive in this respect.

Marked growth stimulation of the microorganisms studied was obtained when certain long chain fatty acids were added to the culture medium in the form of their water-soluble esters, or in admixture with adequate amounts of serum albumin.

Abundant growth of the micrococcus resulted from the addition of oleic, linoleic, linolenic, or arachidonic acid (0.0001 to 0.001 per cent) to a mineral medium containing glucose as sole source of carbon; in the case of this microbial species, none of the other substances tested could substitute for these unsaturated fatty acids.

Enhancement of growth of tubercle bacilli was obtained by adding to the medium 0.001 to 0.01 per cent of a variety of fatty acids (saturated or unsaturated) even in the absence of glucose or of any other readily available carbon compound.

These results suggest that long chain fatty acids can affect the growth of different microbial species through different metabolic channels and that, in order to study the mechanism of these metabolic and growth reactions, it is essential to use the fatty acids under conditions where they cannot manifest their toxic properties.