We compared the intraocular pharmacokinetics of cefazolin with those of cefamandole, a recently marketed cephalosporin with enhanced activity against gram-negative bacilli. Following subconjunctival injection of 12.5 mg into infected eyes (S. aureus endophthalmitis) of pigmented rabbits, both drugs reached peak concentrations greater than 100 microgram/gm in cornea, sclera, and choroid-retina. The half-life was markedly shorter in sclera and choroid-retina than in cornea. Levels in the aqueous humor rose and fell more slowly than those in ocular tissues, reaching a maximum of only 5 to 10 microgram/ml. The pharmacokinetics of the two drugs were virtually identical in most intraocular sites. When cefazolin, which was less irritating than cefamandole by the subconjunctival route, was given in a dosage of 100 mg, levels in ocular tissues were increased by twofold to fourfold and in aqueous humor by 15-fold, compared to the concentrations produced by the 12.5 mg dosage. Levels in the vitreous humor were exceedingly low with both drugs; mean peak concentrations were 0.24 microgram/ml after the 12.5 mg dosage of cefamandole and less than 1.6 microgram/ml after the 100 mg dose of cefazolin.