Radiocarbon (14C) dating of protein-containing material (collagen, hair/wool, silk, leather) contaminated with extraneous carbon (e.g. humic substances) might result in unreliable dates when pretreated with the conventional but inadequate protocols. In this study cross-flow nanofiltration was applied to pretreated, protein-containing material. This method is able to remove low-molecular and high-molecular weight contaminants as demonstrated in previous studies. The sample quality improvement by cross-flow nanofiltration is verified by measuring the C:N ratio before and after nanofiltration. If the C:N ratio of the permeate (sample after cross-flow nanofiltration) falls within the C:N boundaries for uncontaminated wool/hair, silk and bone collagen, it is assumed to be contamination free. In our study, we focused on wool, silk, and collagen samples of known historical age. All samples treated by cross-flow nanofiltration effectively outputs C:N ratios within the expected range and yield for more accurate 14C date in agreement with historical expectations whereas bulk samples with C:N ratio out of the expected ranges, give either younger or older dates. We thus highlight both that C:N ratio is a good indicator of contamination and that cross-flow nanofiltration is an efficient method to treat protein-containing materials prior to 14C dating.