In contrast to Staphylococcus aureus and coagulase-negative staphylococci, group A streptococci are infrequently present on normal human skin, except in certain populations with endemic impetigo. This has been attributed to differences in susceptibility to the bactericidal effect of skin surface lipids, particularly unsaturated fatty acids. When an M type 6 strain group A streptococcus was exposed to 500 mug of oleic acid per ml, viable counts decreased by 4 logs in 5 min. The rank order of killing was 35 > 20 > 4 degrees C. Oleic acid did not kill a strain of S. aureus, a strain of coagulase-negative staphylococcus, or a strain of Escherichia coli, but bound rapidly to these bacteria as well as to the group A streptococcus. The loss of [(3)H]uridine from labeled oleic acid-treated group A streptococcal cells was greater than 100 times that of controls. There was no loss of [(3)H]-thymidine from group A streptococci or of [(3)H]uridine or [(3)H]thymidine from identically exposed coagulase-negative staphylococci. When [(3)H]uridine was added to group A streptococci during mid-log-phase growth, cessation of uptake occurred within 5 min of addition of 50 mug of oleic acid per ml. Electron microscopic changes seen within 5 min included condensation of the nucleoid and distortion of the streptococcal surface by numerous clumps and blebs. Coagulase-negative staphylococci, S. aureus, and E. coli similarly exposed showed no comparable electron microscopic changes. We propose that oleic acid kills group A streptococci by altering the integrity of the cell membrane with resulting loss of ribonucleic acid but not deoxyribonucleic acid.