Toxicity testing of hydrophobic organic compounds (HOCs') in plastic cell culture plates is problematic due to compound losses through volatilization and sorption to the wells and Culture medium constituents. This leads to poorly defined exposure and reduced test sensitivity. Passive dosing call overcome these problems by the continual partitioning of HOCs from a dominating reservoir loaded in a biologically inert polymer such as silicone, providing defined and constant freely dissolved concentrations and also eliminating spiking with cosolvents. This Study aimed to select a Suitable passive dosing format for in vitro tests ill multiwell plates and characterize its performance at 37 degrees C. Silicone O-rings were the most Suitable format; they were both practical and demonstrated excellent passive closing performance. (1) The rings were loaded by partitioning from a methanol solution containing polycyclic aromatic hydrocarbons (PAHs) (log K(OW), 3.33-6.43) that served as model compounds, followed by removal of the methanol with water. This resulted in highly reproducible HOC concentrations in the silicone O-rings. (2) The release of PAHs into aqueous solutions was rapid and reproducible, with equilibrium partitioning being reached within hours. (3) The buffering capacity of the O-rings was sufficient to maintain Stable concentrations over more than 72 h. The O-rings were then applied to test a range of PAHs at their aqueous Solubility in all array of established in vitro cell culture assays with human cells and cell lines. These included the formation of reactive oxygen species, induction of the IL-8 cytokine promoter, and secretion of MCF-1 by the cells. The biological responses depended oil the melting point of the individual PAHs and their maximum chemical activities (a(max)), Only those PAHs with the highest a(max) stimulated the formation of reactive oxygen species and MCP-1 secretion, while they inhibited the induction of the IL-8 cytokine promoter.