We administered cefamandole nafate to 69 patients before cataract extraction and assayed serum and aqueous humor for antibiotic content at the time of surgery. Average aqueous humor levels of 0.33, 0.59, 0.21, and 0.14 microgram/ml were achieved at one half, one, two and four hours, respectively, after a 1-g intravenous dose. After a 2-g intravenous dose, we found average aqueous humor levels of 1.26, 1.0, 1.57, 0.73, and 0.34 microgram/ml at one half, one, two, four and six hours, respectively. Therapeutic levels in primary aqueous humor effective against common grampositive pathogens were consistently achieved with the 2-g dose, but levels effective against Enterobactereaceae were attained only erratically.