In the accompanying paper, we showed that natural killer (NK) cells were a major population in the naive spleens of normal mice that responded directly to a T cell growth factor, interleukin 2 (IL 2), and clonally replicated without other stimulating agents. The cloned cells growing in IL 2 showed a potent NK activity against several NK targets without addition of an NK-activating agent, interferon (IFN). In the present study, therefore, we examined whether these cloned NK cells on their own produced IFN. It was found that all NK clones growing in IL 2 produced IFN in the culture fluids. The titers of IFN produced in the IL 2-containing media correlated well with the number of growing cells. With the culture in the absence of IL 2, neither cell growth nor IFN production could be detected. Addition of Con A into the culture in the IL 2-free media showed no IFN production. The antiserum neutralizing IFN alpha and IFN beta failed to significantly neutralize IFN produced by NK clones. Treatment with either a pH of 2.0 or antiserum neutralizing mouse IFN gamma resulted in a marked reduction of IL 2-induced NK IFN, indicating that a major part of IFN produced was IFN gamma. These results indicate that IL 2 stimulates NK clones to proliferate, accompanied by IFN gamma production. The results also show that an NK clone, when stimulated with Sendai virus, produced a type 1 IFN (IFN alpha and/or IFN beta), suggesting that murine NK cells can produce both type 1 (alpha and/or beta) and type 2 (gamma) IFN, depending on inducers.