Both eyes of rabbits whose right eyes had previously been injected with Neisseria meningitidis were injected with Diplococcus pneumoniae. The pneumococci failed to grow in the right eye of most rabbits challenged 2 and 4 days after the injection of the meningococci, but grew well in all left eyes. The "protective" effect was less pronounced in the rabbits challenged on days 6 and 9 and virtually absent when the interval between the two injections was 14 or 22 days. Pneumococci grew equally well in both eyes of rabbits that had received nitrogen mustard treatment prior to the intravitreal injection of meningococci, indicating a possible role for the polymorphonuclear leukocyte in the protective effect.