The susceptibility of newborn and infant mice to eye infection by Pseudomonas aeruginosa was studied in 5-, 10-, 15- to 16-, and 21-day-old mice. In the first of three age-related susceptibility experiments, inoculation of P. aeruginosa under the unopened eyelids of infant (5- and 10-day-old) mice in the absence of prior corneal wounding resulted in acute infection and rapid death of many of the animals. However, endophthalmitis was observed in about 30% of bacteremic animals that survived to age 14 to 15 days. In the second experiment, 15- to 16-day-old mice whose eyes were open received P. aeruginosa topically onto either wounded or unwounded corneas. At least 50% of the mice that received both corneal wounding and the bacteria exhibited keratitis, endophthalmitis, and subsequent phthisis bulbi. None of the infected mice died of bacteremia. In addition, mice infected in the absence of corneal wounding did not exhibit any eye damage. In the third experiment, the wounded-cornea responses of 21-day-old mice to P. aeruginosa were more variable. Thirty seven percent of the mice exhibited an intermediate response of decreased eye size and cataracts which was not observed in 15- to 16-day-old mice, 32% recovered spontaneously, and 29% exhibited complete shrinkage of the infected eyes. The variability of the latter responses may reflect a transitional maturation period of natural immunity to the organism in some of the animals, since all 4- to 6-week-old adult mice respond routinely to ocular wounding and similar infections with the organism by undergoing a spontaneous resolvable keratitis (3 to 4 weeks).