Molecular species that are immobilized in discrete locations on a planar support such as protein bands on a gel or a blotting membrane or species applied in dots or spots on a membrane are reacted with binding reagents that are applied through a porous hydrophilic transfer sheet placed over the planar support, the sheet having at least one region that is laterally bordered by a barrier with the binding reagent retained within the bordered region. The bordered region is placed directly over an area on the planar support where the molecular species are expected to reside if they are present on the support. The binding reagent is then delivered into the support to contact the species. Targeted delivery of the binding reagent is thus achieved with improved efficiency.