In the present study we investigate the relevance of the concept of underspecified inflection markers for the processing of language in the human brain. Underspecification is recognized as the main source of syncretism in many current morphological theories. However, relatively little is known about its cognitive status. In underspecification-based theories, a competition among morphological exponents arises systematically. In order to win such a competition, an inflection marker has to meet two requirements: COMPATIBILITY and SPECIFICITY. If underspecification is real, these two principles should also be an inherent part of the language processing system. One should therefore be able to observe separable effects for the violation of each of the criteria. We used the event-related potential (ERP) violation paradigm to test this hypothesis in the domain of strong adjective inflection in German. We expected differences in brain potentials between two incorrect conditions whenever they represented different types of violation (of compatibility and specificity). Our findings strongly support underspecification: an ERP-component related to morphosyntactic integration (viz. left anterior negativity; LAN) was modulated by violations of specificity versus compatibility. Furthermore, the neurophysiological evidence helps to distinguish between two kinds of morphological underspecification that have been proposed: it argues for maximal rather than minimal underspecification. Finally, the observed brain responses indicate increased processing demands for highly specific markers, which suggests that LAN effects may be sensitive not only to morphosyntactic violations but also to the degree of processing effort.