To determine the effect of picolinic acid on NOㆍ production, murine macrophages were incubated with either medium, various concentrations of picolinic acid, or IFN-${gamma}$ plus picolinic acid for 48 hr. Picolinic acid does not induce NOㆍ production by itself, it acted synergistically with INF-${gamma}$ for the induction of reactive nitrogen intermediate production in murine macrophages. Thymidine incorporation appeared to be reciprocally related to nitrite levels, suggesting that IFN-${gamma}$ plus picolinic acid induced NOㆍ synthesis exerted antiproliferative effects.