A methodology for measuring public agricultural research capital is developed and described for the first time, new public agricultural research capital measures for each of the 48 contiguous US states, 1970-1999, are presented, and a new econometric analysis of the contribution of public agricultural research capital to state agricultural productivity is reported. Public agricultural research capital across the states is shown to have five different growth patterns, only one of which is at a constant rate. New TFP results show that public agricultural research capital contributes significantly to agricultural productivity and is larger than previous estimates. Intrastate and spillin public agricultural research capital are shown to be complementary, but private agricultural research capital and public extension are substitutes. The marginal social rate of return to public agricultural, including significant interstate spillovers, is large which has science policy implications.