In order to generate toxicological data of cigarette aerosol different smoke exposure systems have been designed the last years. These machines should simulate smoke exposure conditions comparable to human exposure. Since 1999 we have been optimising the BT020 smoking machine for in vitro testing. Easy handling in practice and high throughput of test cigarettes has been a very important point in the development process. The BT020 is a single port smoking machine directly connected with a novel cell exposure system for Multiwell Cell Culture Plates. A purpose-built smoke distribution device disperses the cigarette smoke across the Multiwell Plate. All wells of the plate are provided with separate smoke distribution and suction ducts. The 'intelligent' computer controlled smoke dilution system allows for precise and rapid dilution of freshly generated cigarette smoke or vapour phase. The machine is applicable for testing cigarettes yielding various amounts of 'tar'. The smoking machine was validated by the uniformity of 'tar' deposition in the wells and by the reproducibility of toxic effects in the Neutral Red Uptake assay (NRU) with human Hep-G2 liver hepatoma cells. The proliferation tests with differently ventilated cigarettes showed reproducible data. Taking into account different smoke dilution factors, toxicological data from 'high tar' cigarettes can be compared with data from 'low tar' cigarettes. An additional advantage is the simple preparation of cells prior to smoke exposure. For 96 Multiwell Plates in particular we use a simple method to assure that cells are only covered by a thin layer of fluid like lung epithelial cells. The cells show different toxic reactions with 'whole smoke' and vapour phase of cigarettes, indicating a significant contribution of the particulate phase.