Abstract: The invention relates to a novel method used for deter-mining the aggressiveness and/or invasiveness of a tumor. The invention also relates to a method for identifying an anticancer agent that makes it possible to reduce the aggressiveness and/or invasiveness of a tumor. The invention is based on the inventors' findings that vimentin produced by the cancer cells can be a marker of the aggressiveness and/or invasiveness of a tumor. This maker capacity is not associated with expression of the vimentin in the cancer cells but is with a post-translational transformation of this different protein according to the degree of aggressiveness and/or invasiveness.