The immune system is believed to be a sensitive indicator for adverse polychlorinated biphenyl (PCB)-induced health effects. Four commercial PCB mixtures (Aroclors) or six individual PCB congeners were evaluated for their effect on splenocyte viability and lipopolysaccharide (LPS)-induced splenocyte proliferation in vitro in two strains of mice, C57B1/6 (high affinity aromatic hydrocarbon receptor (AhR) complex) and DBA/J (low affinity AhR complex). All four Aroclors, the selected individual noncoplanar congeners, or two tertiary mixtures containing one congener from each class significantly decreased the in vitro LPS-induced proliferation of murine splenocytes in either strain of mice without inducing a significant decrease in viability. In contrast, selected individual coplanar or mono-ortho-coplanar congeners did not inhibit splenocyte proliferation or viability at any concentration. These results suggest that mixtures of PCBs and/or congener class (specifically, noncoplanar congeners) may be more highly immunotoxic than individual planar and mono-ortho-coplanar congeners alone. Thus, this in vitro assay has revealed a more complex pattern of immunotoxicity of Aroclors versus individual congeners than has previously been reported or anticipated based on both in vivo derived immunotoxic data and standard comparisons to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). These results have important practical significance since mixtures of PCB congeners were used industrially and now contaminate the environment.