Abstract: Improved porous materials useful in blotting analyses of proteinaceous and genomic matter are prepared by treating porous substrates with a gas plasma containing at least one monomer. Thus, a porous sheet-like substrate is modified by means of a deposit of a plasma polymerizate on at least one of its surfaces, whereby this modified surface exhibits reduced nonspecific binding of chemical probes in a blotting analysis. By means of a controlled deposition of the plasma polymer at a plasma composite parameter W/FM of less than 1.0.times.10.sup.9 Joules/kilogram, improved porous materials are obtained having increased signal-to-background ratios and enhanced sensitivities in genomic analyses.