Plate diffusion tests showed that nitrofurantoin antagonized the activity of nalidixic acid, norfloxacin, ciprofloxacin and enoxacin against many Gram-negative bacilli, including all Proteus mirabilis and Klebsiella aerogenes strains tested. No interaction was exhibited when nitrofurantoin and the newer quinolones were tested against Pseudomonas aeruginosa, staphylococci or streptococci. Antagonism was not reliably demonstrated in chessboard titrations, especially with the newer quinolones. Continuous turbidimetric monitoring revealed that antagonism in Pr. mirabilis was associated with abolition of the bacteriolytic response to quinolones and parallel viable counts established that the bactericidal effect of quinolones was suppressed. Apparent potentiation of the effect of nitrofurantoin by nalidixic acid and other quinolones against Pr. mirabilis appeared to be caused by inhibition of swarming into the nitrofurantoin inhibition zone.