Rabbit interferons from various sources could be differentiated by filtration through Sephadex G-100 and their molecular weights estimated by comparison with elution profiles of a series of marker proteins of known molecular weight. Each of four different preparations of rabbit interferons was found to contain more than one molecular component. Elution peaks for three NDV-induced interferons were equivalent to the following molecular weights: RK 44,000–45,000 and > 134,000 (variable and < 1% when present); macrophage 37,000, 44,000–45,000, and > 134,000 (variable and <1% when present); and serum 50,000–52,000 and > 134,000 (10% and heat labile). NDV-induced serum interferon may also contain another molecular component of mol wt 45,000 represented by a trailing shoulder from the major 51,000 mol wt peak.

Endotoxin-induced macrophage interferon proved to be polydisperse. Sephadex filtration of this interferon did not reveal clear and consistent elution patterns, partially owing to its low initial titer and lability. However, variable peaks of biological activity could be detected in Sephadex fractions equivalent to approximate molecular weight values of > 134,000, 72,000–78,000, 33,000–38,000, 28,000–30,000, and possibly a component of 42,000–45,000. A major component of mol wt 37,000 was present in all samples of endotoxin-induced macrophage interferon. The other constituents may be biologically active subunits or polymers.

These data indicate that rabbit macrophages produce two primary kinds of interferon: (a) an RK-like component of mol wt 45,000 that is synthesized in greatest amount after viral induction, and (b) a different species of mol wt 37,000 that can also be synthesized in the absence of viral induction. The presence of major interferon constituents of mol wt 51,000 and > 134,000 in rabbit serum after viral induction suggests that macrophages are not the principal interferon-producing cells that respond to intravenous injection of NDV.