The susceptibility of 242 strains of Staphylococcus aureus and 117 strains of streptococci of groups A, B, C, and G to decanoic, dodecanoic, octadeca-9,12-dienoic, and octadeca-9,12,15-trienoic acids was estimated by determination of minimal inhibitory concentrations. S. aureus strains appeared to be generally less susceptible to all four fatty acids than streptococcal strains of all groups. Dodecanoic acid was the most inhibitory fatty acid against both staphylococci and streptococci. Both saturated fatty acids used were more active than the unsaturated acids. Among the unsaturated acids, octadeca-9,12,15-trienoic acid appeared to be more inhibitory for Staphylococcus and Streptococcus strains than octadeca-9,12-dienoic acid. No differences in susceptibility to fatty acids among staphylococcal and streptococcal strains isolated from skin, throat, or other sites were found.