The susceptibility of young chickens to infection with an isolate of Mycobacterium paratuberculosis which had been recovered from diseased tissues of a patient with Crohn's disease was determined. Two-week-old Leghorn-Cochin chicks were inoculated with strain Linda. Six birds received 10(7) organisms orally, six intraperitoneally, and five intracardially. Six uninoculated birds served as contact controls. Birds from each group were necropsied at two-week intervals. Focal granulomatous lesions occurred in two of six chickens which were inoculated orally, in six of six inoculated intraperitoneally, in three of five inoculated intracardially, but in none of six controls. Of the 11 birds with lesions, acid-fast bacilli were demonstrated in five. Immunoperoxidase reactivity paralleled the presence of acid-fast bacilli.