A process is provided for the isolation of a blood polypeptide fraction or derivatives thereof herein termed the "tumor complement fraction" or "TCF", from mammalian blood. The process comprises a series of centrifugal separations of blood fractions under conditions which maintain the integrity of the desired material as it existed in vivo.The Tumor Complement Fraction, is a new material and it constitutes another aspect of the disclosure. Isolated TCF, a natural polypeptide or derivative thereof, is useful as a neoplastic tissue remission and prevention agent.