Resistance to nalidixic acid was comparatively uncommon among clinical isolates of Gram-negative bacteria being present in only 3% of Escherichia coli strains and in 9% of all strains. Among Gram-negative bacteria resistant to nalidixic acid, Proteus mirabilis, Klebsiella, Enterobacter and Pseudomonas species were more common. Transferable resistance to a minimum level of nalidixic acid (10 μg/ml) was not demonstrated in any of 483 resistant strains investigated here or in 50 strains studied earlier. Possible explanations for the apparent absence of R-factor mediated resistance to nalidixic acid are discussed.