Peroxisome proliferator-activated receptors (PPARs) are ligandactivated nuclear receptors. Three subtypes of PPARs (␣, , and ␥) have been identified in different tissues. PPAR␣ and PPAR␥ ligands inhibit cell proliferation and induce differentiation in several human cell models. We demonstrated that both PPAR␣ (clofibrate and ciprofibrate) and PPAR␥ ligands (troglitazone and 15 deoxyprostaglandin J2, 15d-PGJ2) inhibited growth, induced the onset of monocytic-like differentiation, and increased the proportion of G 0 /G 1 cells in the HL-60 leukemic cell line. Moreover, 3 days after the treatment with 2.5 M 15d-PGJ2, an increase in sub-G 0 /G 1 population occurred, compatible with an induction of programmed cell death. To clarify the mechanisms involved in HL-60 growth inhibition due to the effects of PPAR ligands, we investigated their action on the expression of some genes involved in the control of cell proliferation, differentiation, and cell cycle progression such as c-myc, c-myb, and cyclin D1 and D2. Clofibrate (50 M), ciprofibrate (50 M), and 15d-PGJ2 (2.5 M) inhibited c-myb and cyclin D2 expression, whereas they did not affect c-myc and cyclin D1 expression. Only troglitazone (5 M) decreased c-myc mRNA and protein levels, besides decreasing c-myb and cyclin D2. The down-regulations of c-myb and cyclin D2 expression represent the first evidence of the inhibitory effect exerted by PPAR ligands on these genes. Moreover, the inhibition of c-myc expression by troglitazone may depend on a PPAR-independent mechanism.