Multiwalled carbon nanotubes (MWCNTs) have been employed for the first time as sorbents for the isolation of basic proteins from other protein species in biological sample matrices by solid-phase extraction (SPE). A microcolumn packed with MWCNTs was incorporated after appropriate pretreatment into a sequential injection system, which facilitates online selective sorption of basic protein species (hemoglobin and cytochrome c in this particular case). The retained protein species were afterwards separated from each other by sequential elution from the microcolumn through the employment of appropriate eluents. A 0.025 mol L(-1) phosphate buffer solution of pH 8.0 facilitated the efficient collection of hemoglobin, while a 0.5 mol L(-1) NaCl solution ensured the quantitative recovery of the retained cytochrome c. With a sample loading volume of 2.0 mL, enrichment factors of 11 and 15 were derived for hemoglobin and cytochrome c, along with retention efficiencies of 100% for both species and recovery rates of 98 and 90%, respectively. A sampling frequency of 8 h(-1) was achieved, and the precisions were 3.0% and 0.8% (RSD) for hemoglobin and cytochrome c at a concentration of 5.0 microg mL(-1). The practical applicability of this system was demonstrated by processing of human whole blood for isolation of hemoglobin, and satisfactory results were obtained by assay with SDS-PAGE.