Movements in total quantity and in quality-adjusted price suggest a supply-side shock in the American automobile market in 1955. This paper tests the hypothesis that the shock was a transitory change in industry conduct, a price war. The key ingredients of the test are equilibrium models of oligopoly under product differentiation. Explicit hypotheses about cost and demand are maintained while the oligopoly behavioral hypothesis is changed from collusive to competitive (Nash) equilibrium. In nonnested (Cox) tests of hypothesis, the collusive solution is sustained in 1954 and in 1956, while the competitive solution holds in 1955. The result does not appear to be an artifact, since it is robust in tests against alternative specifications.