The effects of technical grade pentachlorophenol (T-PCP) exposure on several immunological parameters were examined in adult C57Bl/6 mice following eight weeks of dietary exposure. Immune function tests included mitogen-induced lymphocyte blastogenesis, mixed lymphocyte reactivity (proliferation and cytotoxicity), spontaneous and boosted levels of natural killer (NK) cytotoxicity, and phagocytic activity of resident, thioglycollate-induced, and P815-tumor activated peritoneal macrophages. Thymic and splenic weights, spleen cellularity, percentages of splenic T and B cells, and bone marrow cellularity were also determined. The only statistically significant functional alteration observed in T-PCP exposed mice in these studies was a reduction in the lymphoproliferative response in mixed lymphocyte culture which occurred in the absence of any apparent effect on the generation of cytotoxic cells. Mitogen responses, NK cytotoxicity and macrophage phagocytosis were unaltered by exposure to T-PCP. No changes were observed in spleen or thymus weights or in spleen or bone marrow cellularity. A dose-responsive trend toward reduced T cell and increased B cell percentages in the spleen of T-PCP exposed mice was noted. The apparent functional resistance of T cells, macrophages, and NK cells to T-PCP is in contrast to the marked sensitivity of the humoral immune response to T-PCP induced suppression. The results are discussed in relation to the dioxin contaminants present in T-PCP.