1) An antagonistic effect was observed between nalidixic acid and certain nitrofurans against the organisms of Proteus- Providence group by means of agar diffusion method with sensitivity disc and agar dilution method.
2) This effect was one way, i.e., the antibacterial activity of nalidixic acid was decreased by certain nitrofurans, while that of the latter was not altered by the former.
3) This phenomenon appeared only when the nitrofurans were added to the culture before the addition of nalidixic acid or almost simultaneously together with nalidixic acid.
4) This phenomenon was common to each species of Proteus-Providence group, i.e., P. mirabilis, P. vulgaris, Morganella, Rettgerella and Providencia.
5) Among the nitrofurans, furazolidone, nitrofurazone and guanofuracin exhibited the antagonistic effect, but dihydroxymethylfuratrizine, furaltadone and panazone did not.
6) According to the microscopic observation on the bacterial cells the addition of the nitrofurans appeared to inhibit the cellular elongation resulting by the inhibitory effect of nalidixic acid on the binary fission of the cells.
7) Compared to the single administratiion of nalidixic acid, the combined administration with nitrofurantoin seemed to reduce the antibacterial activity against Proteus-Providence group in urine.