Suspensions of Campylobacter jejuni became non-culturable after storage in sterilized pond water at 4 degrees C for periods between 18 and 28 d, depending on the strain. Suspensions of four strains of C. jejuni that had been in water for 6 weeks, and shown to be non-culturable, were fed to suckling mice. Colonization of mice was established with two of the strains and failed with the other two strains. Examination of these suspensions under the electron microscope showed some cocci having the appearance of being viable, but most cocci and all remaining spiral forms showed extensive degeneration. The results indicate that non-culturable coccal forms of C. jejuni are capable of infecting mice but that this property may differ between strains.