The research described herein evaluates the expression and functional significance of peroxisome proliferator activator receptor-γ (PPAR-γ) on B-lineage cells. Normal mouse B cells and a variety of B lymphoma cells reflective of stages of B cell differentiation (eg, 70Z/3, CH31, WEHI-231, CH12, and J558) express PPAR-γ mRNA and, by Western blot analysis, the 67-kDa PPAR-γ protein. 15-Deoxy-Δ 12, 14-PGJ 2 (15d-PGJ 2), a PPAR-γ agonist, has a dose-dependent antiproliferative and cytotoxic effect on normal and malignant B cells as shown by [3 H] thymidine and 3-[4, 5-dimethylthiazol-2-yl]-2, 5-diphenyltetrazolium bromide assays. Only PPAR-γ agonists (thiazolidinediones), and not PPAR-α agonists, mimicked the effect of 15d-PGJ 2 on B-lineage cells, indicating that the mechanism by which 15d-PGJ 2 negatively affects B-lineage cells involves in part PPAR-γ. The mechanism by which PPAR-γ agonists induce cytotoxicity is via apoptosis, as shown by annexin V staining and as confirmed by DNA fragmentation detected using the TUNEL assay. Interestingly, addition of PGF 2α, which was not known to affect lymphocytes, dramatically attenuated the deleterious effects of PPAR-γ agonists on B lymphomas. Surprisingly, 15d-PGJ 2 induced a massive increase in nuclear mitogen-activated protein kinase activation, and pretreatment with PGF 2α blunted the mitogen-activated protein kinase activation. This is the first study evaluating PPAR-γ expression and its significance on B lymphocytes. PPAR-γ agonists may serve as a counterbalance to the stimulating effects of other PGs, namely PGE 2, which promotes B cell differentiation. Finally, the use of PGs, such as 15d-PGJ 2, and synthetic PPAR-γ agonists to induce apoptosis in B-lineage cells may lead to the development of novel therapies for fatal B lymphomas.