Mouse interferons of three size classes (A, 35,000 to 40,000 daltons; B, 26,000 to 33,000 daltons; and C, 20,000 daltons) were purified from Ehrlich ascites tumor cells infected with Newcastle disease virus. The sequences of the first 24 amino acids (No. 17 has not been identified) of interferons A and B are identical. The sequence of the first 20 amino acids of interferon C differs from that of A and B in 18 positions. There is partial homology in amino terminal sequence between mouse interferons A (or B) and a human fibroblast interferon and between mouse interferon C and a human lymphoblastoid interferon.