The human milk oligosaccharide 3'-sialyllactose (3'SL) has previously been shown to activate murine dendritic cells (DC) in a Toll-like receptor (TLR) 4-mediated manner ex vivo. In this study we aimed to investigate whether 3'SL has similar immunomodulatory properties on human DC. 3'SL was shown to induce NF-κB activation via human TLR4. However, LPS was detected in the commercially obtained 3'SL from different suppliers. After the removal of LPS from 3'SL, we studied its ability to modify DC differentiation in vitro. In contrast to LPS and 3'SL, LPS-free 3'SL did not induce functional and phenotypical changes on immature DC (iDC). iDC that were differentiated in the presence of LPS or 3'SL showed a semi-mature phenotype (i.e., fewer CD83+CD86+ DC), produced IL-10 and abrogated IL-12p70 and tumor necrosis factor-alpha levels upon stimulation with several TLR ligands. Differentiation into these tolerogenic DC was completely abrogated by LPS removal from 3'SL. In contrast to previous reports in mice, we found that LPS-free 3'SL does not activate NF-κB via human TLR4. In conclusion, removing LPS from (oligo)saccharide preparations is necessary to study their potential immunomodulatory function.