HL-60 cells were treated with 100 nm PMA for 24 hr, with 500 μm Bt₂cAMP for 48 hr, or with 500 μm Bt₂cAMP for 48 hr followed by a 24-hr exposure to 100 nm PMA. Control cells were cultured in the absence of any differentiating agent. RNA, isolated from each sample of cells, was subjected to semiquantitative RT-PCR analysis using primers for the human P_2UR cDNA, the human FPR cDNA, and GAPDH cDNA. Each PCR was supplemented with [³²P]dCTP to label the amplified DNA products. After electrophoresis, each PCR band was excised, melted, and analyzed by liquid scintillation counting. The total number of cpm of ³²P that were associated with each RT-PCR product is listed, together with normalized values that were calculated as the ratio: ³²P in PCR product from differentiated cells/³²P in PCR product from control cells.