Fourteen female Holstein calves were obtained from farms with no serological evidence of M. paratuberculosis infection based on serum antibody assays on 100% of adult cattle in the herd. At the age of 4 weeks, 5 calves were orally challenged with 10⁸M. paratuberculosis by mixing the organism in milk for their evening feeding on 3 consecutive days (high dose group). Another 5 calves were similarly challenged 6 times with 10⁶M. paratuberculosis (low dose group). Four non-challenged calves served as controls. Every 4 weeks calves were sampled. Serum antibodies were measured using the M. paratuberculosis antibody Test Kit (IDEXX Laboratories, Inc.). The gamma interferon response of peripheral blood leukocytes stimulated with M. bovis or M. avium PPD was determined using the M. paratuberculosis Gamma-Interferon Test Kit (IDEXX Laboratories, Inc.). Fecal samples were decontaminated with 1.0% HPC and M. paratuberculosis growth was detected using a modified BACTEC radiometric culture system as previously described. At 12-13 months post-challenge, a 1 x 2 cm ileum biopsy and a regional lymph node were surgically obtained from all 14 calves and processed for histopathology and radiometric culture. Four of the 5 high dose calves and 3 of 5 low dose calves have been confirmed infected by isolation of M. paratuberculosis from fecal or tissue samples. The study is ongoing and this abstract represents an interim report of results through 18 months post-challenge. No calves have tested positive for serum antibody to M. paratuberculosis. No calves have tested positive by the gamma interferon assay, based on evaluation of bovis/avium ratios. Radiometric fecal cultures have been positive sporadically. Specifically, 9 of 100 fecal attempts have been positive for high dose calves and 5 of 100 fecal culture attempts have been positive for low dose calves.