Objective: This investigation evaluated the leakage (diffusion) of lipopolysaccharide and dextran beneath cast-gold crowns luted with one of three cements. Method and materials: Thirty extracted molars were prepared for crowns. Thirty crowns with access ports (facial or lingual) were cast in gold and luted to their preparations with zinc phosphate, glass-ionomer, or an adhesive resin cement (10 per group). Teeth and crowns with filters inserted in the ports were immersed in a solution of labeled macromolecules (tetramethylrhodamine isothiocyanate-labeled lipopolysaccharide and fluorescein isothiocyanate-labeleddextran) and evaluated for leakage at 2 weeks and 1, 2, 3, 4, 5, and 6 months. Filters were retrieved and analyzed with microfluorimetry. Results: No filters retrieved from any crowns demonstrated any detectable leakage, and all filters were negative for both dextran and lipopolysaccharide for all evaluation periods. Conclusion: For the 6 months of this investigation, zinc phosphate, glass-ionomer, and adhesive resin cements were equally effective at preventing leakage of detectable molecular concentrations of lipopolysaccharide and dextran to the level of the access ports in the absence of thermal or mechanical stress.