This paper proposes a cointegration approach to testing the validity long-run equilibrium in production, where capital and labour are taken as quasi-fixed inputs. Previous studies consider only capital as the quasi-fixed input and do not take account of the time series properties of the variables, assuming implicitly that they are stationary. The canonical cointegrating regressions (CCR) procedure is employed to test for cointegration in both the single-equation and the seemingly unrelated regressions framework, and long-run equilibrium conditions are tested. The evidence from US manufacturing reveals that capital and labour are not fully adjusted to their long-run optimal values, casting doubt on the long-run equilibrium hypothesis. Copyright K 6 4 A theoretical discussion of a short-run model based on the variable cost function is provided by Brown and Christensen (1981); see also Caves, Christensen, and Swanson (1981). 5 This specification of the stochastic cost function is an adaptation of McElroy (1987) and Brown and Walker (1995). Their analysis, however, rests on the long-run cost function and does not allow for stochastic properties of the time series. 8 Thus long-run equilibrium is viewed here as an average relationship. Kulatilaka (1985), however, views it as holding for every observation. 9 This study probably represents the first real application of the system-based CCR procedure within the context of SUR. Ogaki and Park (1998) and Ogaki (1992) also apply the system-based CCR procedure, but in a slightly different form mainly to deal with a cointegrated system with a trend stationary process. For applications of the single-equation CCR procedure, see Fisher and Park (13 Park (1990, p. 119) shows that the asymptotic distribution of the Wald tests using random walk processes is 2 distributed asymptotically. Referees, however, point out that this result could be interpreted as ad-hoc evidence, since the asymptotic behaviour of the simulation-based tests is not yet examined. 14 The H(0,1) test indicates that the null hypothesis is not rejected for all cases, except for a pair of ln P k and ln P Ł K for which the null is marginally rejected at the 5% level, but not at the 2.5% level.