SummaryCutaneous photosensitivity reactions are a consistent although uncommon feature of the fluoroqui‐nolone group of antihiotics, which are related to nalidixic acid. Ohjective laboratory and clinical data are now routinely required by regulatory bodies for new drugs suspected of being photosensitizers. but no clear recommendations exist. A series ofin vitrotests ranging in complexity revealed a UVA‐dependent phototoxic potential for the fluoroquinolone norfloxacin similar to that for ciprofloxacin. and less than that of nalidixic acid. Controlled monochromator phototesting, designed to reveal the clinical characteristics, wavelength dependence and severity of cutaneous reactions in normal subjects showed both norfloxacin and ciproHoxacin to have a weak phototoxic potential which clears within 4 weeks of stopping the drug. UVA wavelengths (335±30 nm: 365±30 nm) appear most responsible for producing an asymptomatic erythema which is maximal at 24 h.The clinical study differs from those used previously in being blind, containing positive and negative controls, and phototesting after cessation of drug intake. The methodology has the anticipated limitation of failing to detect idiosyncratic photosensitivity resp