Dialysates containing transfer factor (TF) activity were prepared from lyzed splenic cells of SPF mice immunized with live, peripherally avirulent Langat virus (TP21 E5 "14" clone) from the tick-borne encephalitis (TBE) complex. The amount of TF was estimated by its capacity to generate in recipient inbred C3H mice cytotoxic T lymphocytes inducing lysis of TBE virus-infected target cells as demonstrated by 51Cr--release assay. A 100 to 1000-fold concentration of TF activity was achieved by combination of the two-step ethanol precipitation of crude dialysates with subsequent fractionation on Sephadex G-25 by exclusion chromatography. Materials from individual concentration steps showed reduced amounts of admixtures, as revealed by absorbance profiles of their chromatograms. In the final product the protein content was most decreased.