In order to study the different antigenic components recognized by sera from paratuberculosis goats, sera from a) goats considered pathologically infected by M. paratuberculosis (10); b) goats (4) and rabbits (4) hyperimmunized with PPA-3* and a goat strain of M. paratuberculosis and c) ELISA positive goats (10), were analyzed by western-blotting using PPA-3 as antigen. More than ten different major antigenic components were recognized by the sera in all the goats analyzed. A specially concentrated antigenic cluster appeared between 45-35 Kd. This cluster is almost invariable in all positive goat sera studied, and it was not present in healthy or negative animals. A specific antigen of around 65 Kd was present in only some sera from pathologically infected goats. This antigen was not removed by absorption with M. phlei and was not identified in the sera from rabbits nor goats immunized with PPA-3, although it was present in goats immunized by M. paratuberculosis and in some sera from ELISA positive animals. This antigen was recognized by 7 of 28 goats studied. This antigen could be related to the 65 Kd antigen of other mycobacteria and its presence in only some animals may be the result of an autoimmune disease produced in some animals infected with M. paratuberculosis.