We have measured the electrical transport along individual lambda-DNA molecules stretched across a set of parallel nanoelectrodes using the four-probe method. Up to a relative humidity (RH) of 78%, we find no evidence for electrical transport along lambda-DNAs. However, for RHs above 79%, we measure considerable current. The electrical conductance of lambda-DNA increases exponentially with increasing RH. For a long lambda-DNA molecule with a random base sequence, our results indicate that the current flows through dense water layers surrounding DNA and is dominated by ion transport rather than by electron transport through lambda-DNA itself. (C) 2011 Elsevier B.V. All rights reserved.