P-1 was partially hydrolyzed with 0.01, 0.05, and 0.1 M trifluoroacetic acid (TFA), successively, and the dialyzable (E-1, E-2, and E-3) and non-dialyzable (I-1, I-2, and I-3) fractions were prepared and analyzed chemically and immunochemically. Either I-1 or E-1 reacted with anti P-1 serum as strongly as P-1 and were mitogenic. The cross-reactivity of I-2 and I-3 was less than I-1 with anti P-1 serum. However, they were as mitogenic as I-1. The cross-reactivity of E-2 and E-3 to anti P-1 serum was also very weak, and they were not mitogenic. The E-1 fraction had a similar sugar composition to I-1 and P-1. E-2 was a monosaccharide, all of Ara, and would be from the linkage of furanosyl residues in P-1. The composition of E-3 was free from Ara and the structure of E-3 was similar to that of I-3. E-3 would be considered to be deleted arabinofuranose from E-1. These results suggest that the mitogenic activity measured by the alkaline phosphatase assay is a property of the core part, I-3, but that P-1 contains several epitopes other than the core part by the immunochemical analysis.