The cross-reactivities of four quinolone antibiotics--ofloxacin, norfloxacin, ciprofloxacin, and nalidixic acid--toward the EMIT II Opiates enzyme immunoassay were examined. Drug-free urine was spiked with the individual drugs up to 5,000 mg/L. Only ofloxacin showed potential for causing a false-positive opiates immunoassay screening result (apparent morphine > 300 micrograms/L). The method produced a positive opiate result at an ofloxacin concentration of 200 mg/L, a 0.16% cross-reactivity. Three hospitalized patients taking therapeutic doses of ofloxacin all gave false-positive EMIT II Opiates urine screening results. Three patients taking norfloxacin and three patients taking ciprofloxacin gave true-negative urine screening results. False-positive results were also obtained from the urines of two volunteers who each consumed a single 400-mg ofloxacin pill.