We have substituted up to ca. 2 at.% of the Cu in orthorhombic YBa2Cu3O7 by Ag. This substitution leads to a wider temperature range for the resistive transition to the superconducting state and a decrease in the fraction of the sample that is superconductive, as compared to the Ag-free material. Introduction of up to ca. 1.5 at.% Ag causes a slight lattice expansion. Higher nominal levels of Ag bring about the formation of additional phases, which, however, still show zero resistance, albeit at lower temperatures. We use solid state chemical arguments to infer that Ag substitutes for Cu in the CuO4 chains of the structure, in which case our results suggest that optimal superconducting behaviour is obtained when these chains are intact.