Using a panel data set for the forty-eight contiguous states from 1970 to 1983, several estimates are provided of a Cobb-Douglas production function with three types of public capital as inputs. Various specification tests are systematically applied to test for both random and fixed state effects, nonstationarity, endogeneity of the private inputs, and measurement error. In the preferred specification, which is first differences with fixed state effects, the public capital variables are not significant, while the fixed state effects and private input variables are significant.