After a suspension of tumor pieces was grafted into newborn and adult (CBA × C57BL/6J)F₁ and BALB/c mice, the growth of Lewis lung adenocarcinoma and mammary gland adenocarcinoma was inhibited in newborn mice, whereas sarcoma of the rectum (SR-1-75) grew at the same rate in newborn and adult recipients. Neonatal thymectomy stimulated the growth of hepatoma (H-2-73) in newborns. The degree of tumor growth inhibition was age-dependent: The maximum inhibition was observed in 1- to 6-day-old recipients, but later it gradually decreased. The hepatoma (H-2-73) and ovarian carcinoma (OC-1-75) with inhibited growth in newborns and the tumor (SR-1-75) with uninhibited growth had equally low immunogenicity. The data suggested that newborns possess factors that inhibit tumor growth but these factors disappear with increased age of recipients.