L cells in which interferon had been induced by u.v. irradiated Newcastle disease virus were exposed for varying periods of time to [3H]-l-methionine, [3H]- or [14C]-d-glucosamine, [3H]-protein hydrolysate or [14C]-l-fucose. Interferon was subsequently purified by SE-Sephadex chromatography and polyacrylamide gel electrophoresis to contain in excess of 1 × 106 units/mg. of protein. Electropherograms disclosed that methionine, amino acid mixture and, to a lesser degree, glucosamine, were incorporated into proteins associated with the dominant interferon fraction. No incorporation of fucose was noted. Methionine and glucosamine labels were absent from the major interferon band in control preparations from non-induced cells, co-electrophoresed in acrylamide gel with non-labelled chromatographed marker interferon.