The chemical nature of human dialysable transfer factor (TFd), capable of augmenting delayed hypersensitivity (DH) in human recipients, and some mammalian organ dialysates, known to augment DH in antigen-primed guinea pigs, were compared using chromatography on Sephadex G-10 and G-25 columns and on thin-layer plates. The fractions of human leukocyte dialysate which eluted at or after the Vt of the Sephadex columns have previously been shown to contain the in vivo TFd-activity and therefore special attention was paid to corresponding dialysate fractions. All together 52 identified or unidentified components were found at or close to this elution region with thin-layer chromatography (TLC). The 14 identified substances were nucleobases, nucleosides, sugars and aromatic or heterocyclic amino acids. Unidentified components had similar staining characteristics as the identified ones on TLC. No evidence was found for the presence of peptides or nucleotides. There were no components specific for human leukocyte dialysate. Several of the identified and unidentified substances in fractions of humal dialysable leukocyte extract were common to all or nearly all dialysates. The possibility that some of the unidentified components might be responsible for the in vivo effect of human leukocyte dialysate in man or guinea pig is discussed.