1. Extracts prepared from livers, spleens, and lymphatic tumors of leukemic Ak mice, were centrifugated at 1400 × g (15 minutes), then 7000 × g (5 to 10 minutes), and finally passed through Selas porcelain niters. The filtered extracts were then inoculated into newborn (less than 16 hours old) C3H or C3H(f) suckling mice. 2. Of the 84 mice inoculated with fresh, filtered extracts, 9 died from leukemia, and 15 others developed bilateral epitheliomas possibly arising in the salivary glands. Of the 54 control litter-mates inoculated with heated (68°C for 30 minutes) extracts, only one developed a cervical tumor. 3. Extracts prepared from Ak leukemic cells may therefore contain 2 different agents, pathogenic for newborn C3H or C3H(f) mice: one, causing leukemia, and another salivary gland epitheliomas. These agents may differ in size. 4. It is assumed that the leukemic agent may interfere with the tumor agent preventing it from exerting its pathogenic properties on the cells of its usual Ak, carrier host, or that mice of the Ak line may be naturally resistant to the tumor agent. 5. Salivary gland tumors have not been observed in untreated mice of either the Ak, the C3H, or C3H(f) inbred lines.