As evidence on infection by M. paratuberculosis in human beings is increasing, we tested the level of antibodies in a sample of slaughterhouse or related activities workers compared with a control one of administrative workers. The total number of sera were 1312 in the immunodiffusion (ID) test and 1277 in the ELISA. Both methods were carried out with a protoplasmatic antigen following the instructions of the supplier (Allied Laboratories,Inc.). Each sample was submitted to the ELISA in duplicate, using either an anti-IgM or an anti-IgG conjugate. The results were expressed as the ratio of the optical density (OD) of each serum to the OD of a normal reference serum included in each plate. In the ID test 7 sera gave a positive reaction, 2 belonging to the control sample and 5 from the slaughterhouses sample, resulting in 0.53% of seroreactivity in the whole sample. However, 4 of the positives corresponded to a single slaughterhouse, in which the percentage of positives reached 1.52%. In the ELISA, the comparison with the control group indicated a signficiantly higher IgM index in 7 places. For the IgG, out of a total of 6 significant differences, only 2 places showed higher indexes, while the other 4 (poultry slaughterhouses) had lower ones. Five places showed significant differences in both Ig's. In regards to the type of activity, the highest indices corresponded to tripe butchers, veterinarians, slaughterers and transporters for IgM, while for IgG the highest indexes were observed for tripe butchers and veterinarians and the lowest for transporters. Our results showed a reactivity of human beings against antigens of the avium-paratuberculosis group. Although it could be explained by just an environmental contact, the fact that two clear patterns seem to arise from the IgM/IgG reactivity, with the cases of increase in both mostly related to contact with ruminants, leads to suspect some specific relationship with paratuberculosis that seems to deserve a more thorough study.