Studies were conducted to compare the interferon (IFN) production of four strains of mice when injected with Newcastle disease virus (NDV), polyinosinic-polycytidylic acid (poly I. C), lipopolysaccharide (LPS) or purified protein derivative (PPD). When injected with NDV, C57BL/6 and DDD mice generated higher levels of IFN than BALB/c and C3H/He mice. Upon injection with poly I. C or LPS, C57BL/6 and C3H/He mice produced higher levels of IFN than BALB/c and DDD mice. Differences in IFN production were also observed among BCG-sensitized mice of all four strains upon injection with PPD, C57BL/6 mice engendered the highest levels of IFN, while the C3H/He and DDD mice were intermediate in their response, with the BALB/c mice producing the least amount of IFN.