The quantitative extraction of bile acids and their conjugates from serum was investigated using different commercially available reversed-phase bonded silica cartridges. Serum was diluted with 0.1 M sodium hydroxide or 0.5 M triethylamine sulphate, pH 7.0, and heated at 64 degrees C for 20-30 min prior to application to the cartridge. Bile acids were adsorbed to the bonded silica particles, and recovered by elution with a small volume of methanol. Dilution of serum with triethylamine sulphate gave quantitative extraction of bile acids from serum using Bond-Elut C18 cartridges, but the milder conditions of the applied sample compared with dilution with sodium hydroxide permitted re-use of the cartridges with no loss in performance, and gave relatively clean extracts. Large variations in the recovery of bile acids were observed using sorbents from different manufacturers. These studies highlight the need to evaluate critically the performance characteristics of solid-phase cartridges when the manufacturer or batch lot is changed.