Polypeptides of egg-borne Sendai virus (egg Sendai), which is biologically active on the basis of criteria of the infectivity for L cells and of hemolytic and cell fusion activities, were compared by polyacrylamide gel electrophoresis with those of L cell-borne (L Sendai) and HeLa cell-borne Sendai (HeLa Sendai) viruses, which are judged biologically inactive by the above criteria. Densitometer profiles on the stained gels of egg Sendai resolved six polypeptides (virion protein [VP] 1 to VP6), in which VP2 and VP4 were identified as glycoproteins by PAS stain. Comparative electropherograms of both L Sendai and HeLa Sendai revealed that there were significantly larger amounts in the VP2 region of these viruses but VP4 was present only in greatly reduced amounts as compared to egg Sendai. It was also found that VP2 of L Sendai and HeLa Sendai consisted of two components, VP2a and VP2b, but the one of egg Sendai consisted of only VP2a. A mild trypsin treatment which converts both L Sendai and HeLa Sendai to a biologically active form selectively removed VP2b from these viruses and increased concomitantly the amounts of materials in the VP4 region. The same treatment of egg Sendai affected neither its biological activities nor its electropherogram. Consequently, gross polypeptide profiles on the stained gels of L Sendai and HeLa Sendai after trypsin treatment became favorably comparable to that of egg Sendai. Electrophoresis of labeled L Sendai and HeLa Sendai with a ³H-amino acids mixture and ¹⁴C-glucosamine resolved at least three glycoproteins, GP1, GP2, and GP3, each corresponding to VP2a, VP2b, and VP4, respectively. The trypsin treatment of these viruses removed almost all the radioactivity of GP2 and simultaneously increased the radioactive counts of GP3 and raised small amounts of rapidly moving heterogeneous glycoprotein, GP4. A possible relationship between the biological modification and the above characteristic polypeptide patterns of Sendai virus was discussed.