In this study, we have compared the time and dose response curves for TCDD using the pGudLuc1. 1-chemically activated luciferase expression (CALUX) cell bioassay and two new recombinant cell lines that contain a stably transfected mutated form of firefly luciferase reporter gene (pGudLuc 6.1) or enhanced green fluorescent protein (EGFP) reporter gene (pGreen1. 1). The time course of induction with pGudLuc1. 1-containing H1L1. 1c2 cells is transient, with maximal activity observed at 4 hours after treatment with 1 nM TCDD. In contrast, expression of luciferase from the pGudLuc6. 1-containing H1L6. 1c2 cells and the pGreen1. 1-containing H1G1. 1c3 cells progressively increases with time, with luciferase activity increasing at a significant faster rate than that of EGFP. Dose response analysis with each cell line at optimal analysis times reveal similar relative dose response curves and EC_ {50} s for H1L6. 1c2 and H1G1. 1c3 cells, while the EC_ {50} for TCDD in the H1L1. 1c2 cells was about 7-fold lower. In addition, these bioassay systems respond to halogenated and/or polycyclic aromatic hydrocarbons in a dose-specific manner. Given the above differences between cell lines and reporters, the choice of which cell line to use will certainly be dependent on the specific questions and issues being examined.