An enzyme linked immunosorbent assay (ELISA) based method for the determination of transferrin in cerebrospinal fluid (CSF) is described. This method allows transferrin determinations in ultra-small (</= 5 microL) sample volumes. The proposed method offers the possibility to determine transferrin at very low concentrations at which existing methodologies fail because of their insufficient detection limits. The accuracy of the proposed technique was validated by comparing the results of 20 CSF samples obtained by ELISA to those measured by nephelometry yielding mean +/- SD values of 21.1+/-5 mg/L and 20.6 +/- 6 mg/L, respectively (y = 1.13X - 2.97, r = 0.899). The interassay CV was below 10% whereas the detection limit was 2.9 microg/L.