Chromatographically purified fractions of aqueous-ethanolic extracts from Baptisia tinctoria roots contained a strong lymphocyte DNA synthesis-stimulating activity. Electrophoretic analysis of these fractions revealed four distinct protein bands with molecular masses of P1 = 58 kD; P4 = 31 kD; P5 = 26 kD; and P6 = 14 kD. They contained carbohydrate as determined by periodic acid Schiff staining. An estimation of the approximate amount of sugar was done by using human transferrin as a reference, this method revealed the following values: P1 = 27%; P4 = 12%; P5 = 14%; and P6 = 8%. The mixture of proteins and every single band were immunoreactive with a polyclonal antiserum against Baptisia proteins determined in immune and dot blots, respectively. Electrophoretically purified proteins were characterized by tryptic cleavage and determination of their amino acid content. They contained several common amino acids, predominantly aspartic acid, glutamic acid, threonine, and alanine. The content of glucosamine and/or galactosamine was less than 0.2 Mol-per cent. The four proteins revealed pI values between 5.3 and 4.7. Protein P 4 was immunochemically related to phytohemagglutinin but, in contrast to PHA-P, it exhibited no hemagglutinating activity and no leucagglutinating activity like PHA-L.