Extracts of mollusks collected from eight cities along the coast of the Bohai Sea, China, were used to determine dioxinlike activity using an in vitro bioassay approach. Dioxinlike activity of total extracts was detected by measuring luciferase activity in a stable transfected rat hepatoma cell line, H4IIE, containing an aryl hydrocarbon receptor (AhR)-responsive element linked to a luciferase reporter gene. Luciferase activity was expressed as a percentage of the maximum response observed for 2,3,7,8-TCDD (% TCDD(max)). All mollusk samples elicited significant dioxinlike activity except those from Dalian (N1). The greatest magnitudes (+/-SD, data ex N1) of activity observed for these extracts ranged from 4.88+/-0.48% to 11.38+/-1.43% TCDD(max). The mean activity was 7.37+/-1.85% TCDD(max). Concentrations of TCDD-EQs in mollusk extracts were from 227.4 to 547.5 pg g(-1) lipid. The cytotoxic effect of the mollusk extracts on cells was assessed at the same time. Six of the mollusk extracts (N2, N3, N4, S1, S2, and S4) caused noticeable growth inhibition over the exposure period at concentrations higher than 0.33% of the extracts concentrations (0.83 microL extract well(-1)). All the dilutions of sample N1 were cytotoxic to the H4IIE cells.