To study the entrance of M. paratuberculosis in the intestinal mucosa of sheep, ten one month old lambs were orally infected with 5 x 10¹⁰ M. paratuberculosis , ovine strain. In groups of two, they were killed at the following intervals: 12 hours p.i., 36 h.p.i., 3, 7 and 14 days p.i. We used Z-N stain and immunohistochemical technique of ABC-Peroxidase, using a primary antibody against M. paratuberculosis antigens, in samples from the ileocecal valve, ileal Peyer's patch, nine jejunal Peyer's patches and jejunal mucosa located between Peyer's patches. The samples were fixed in formalin and embedded in parafin. Positive results using ABC-Peroxidase technique were found at 3, 7 and 14 d.p.i., the maximum positivity detected at 7 d.p.i.. The antigens of M. paratuberculosis were mainly observed in the M cells of the dome of Peyer's patches. Macrophages subepithelially located in the domes were sporadically positive, and a few positive cells were observed in the epithelium of those villi close to the domes, with high positivity. According to these results, the antigens of M. paratuberculosis detected by immunohistochemical methods but negative by Z-N stain, suggest the presence of L forms, during the initial phases of infection, which would be located in the cytoplasm of M cells, slowly entering across the M cells.