The Dirac and Weyl semimetals are unusual materials in which the nodes of the bulk states are protected against gap formation by crystalline symmetry(1-4). The chiral anomaly(5,6), predicted to occur in both systems(7-10), was recently observed as a negative longitudinal magnetoresistance (LMR) in Na3Bi (ref. 11) and in TaAs (ref. 12). An important issue is whether Weyl physics appears in a broader class of materials. We report evidence for the chiral anomaly in the half-Heusler GdPtBi. In zero field, GdPtBi is a zero-gap semiconductor with quadratic bands(13,14). In a magnetic field, the Zeeman energy leads to Weyl nodes(15). We have observed a large negative LMR with the field-steering properties specific to the chiral anomaly. The chiral anomaly also induces strong suppression of the thermopower. We report a detailed study of the thermoelectric response function alpha(xx) of Weyl fermions. The scheme of creating Weyl nodes from quadratic bands suggests that the chiral anomaly may be observable in a broad class of semimetals.