Numerous articles have appeared in the literature regarding the effect of the prolonged administration of various doses and kinds of antibacterial agents on the bacterial flora of the throat. In 1948 Lipman, Cross, and Boots1demonstrated in throat cultures a change from predominately Gram-positive to predominately Gram-negative flora during the administration of oral penicillin, with a rapid reappearance of a predominately Gram-positive flora after penicillin was discontinued. They found no evidence of the appearance of bacterial resistance to penicillin during its administration. Smith and Bloomfield2in 1948 presented data showing a similar conversion from Gram-positive to Gram-negative flora in the throat during penicillin administration. Haffner, Neter, and Rubin3in 1950 again demonstrated a similar conversion during the administration of both penicillin and sulfonamides. They felt that the children whose flora converted from Gram-positive to Gram-negative organisms during antibacterial therapy showed more severe illness than those who did