A study was undertaken to determine the extent of inactivation of gentamicin, tobramycin, ticarcillin and carbenicillin when given subconjunctivally in amino-glycoside-penicillin combinations. Rabbits were injected with such a combination in either the same or opposite subconjunctival sites. The aqueous humor was aspirated and bioassayed for antibiotic activity at 1 and 4 hours after injection. The influence of the aqueous humour upon antibiotic inactivation was studied by in vitro experiments. Ticarcillin penetrated into the aqueous humour better than carbenicillin. Some chemical inactivation of all the antibiotics given in combination occurred. There was less inactivation of the aminoglycosides at 4 hours when the antibiotics had been injected at opposite poles. The inactivation occurred primarily within the aqueous humour but to a lesser extent in the subconjunctival space. The degree of inactivation is likely of little clinical significance with the doses usually used.