8 páginas, 3 tablas, 5 figuras. ; The differences in pathogenicity between an inoculum derived directly from an intestinal tissue homogenate from a paratuberculosis affected sheep and the S-type Mycobacterium avium subsp. partuberculosis (Map) strain isolated in laboratory media from the mentioned homogenate were assessed in two experiments in lambs. Specific peripheral immune responses were significantly lower in animals inoculated with the cultured organisms that showed only granulomatous lesions in the intestinal lymphoid tissue. However, in the homogenate group, more abundant granulomata also occurred in the lamina propria. Map was isolated only in lambs infected with the culture strain. Map DNA was demonstrated by nested-PCR in all the lambs but in a lower proportion (57.1% vs 100%) in those from the culture group. Under these particular experimental conditions, the results suggest that an attenuation of Map virulence has occurred in the cultured strain compared to the initial tissue homogenate, even after a low number of passages. ; This work was supported by grants AGL2009-05820-C02 and AGL2012-39818-C02-01 from the Spanish Ministry of Economy and Competitiveness. The authors wish to thank all the staff of the IGM (CSIC-ULE) responsible for handling the experimental animals. The technical support of S. Morales is acknowledged. M. Fernández and M. Royo are funded by a predoctoral contract from the “Junta de Castilla y León” and Spanish Ministry of Economy and Competitiveness, respectively. J. Benavides was supported by CSIC through the JAE-Doc program, financed in part by European Social Fund (ESF). ; Peer reviewed