Recent studies indicate that inhaled ultrafine particles can pass into the circulation. To study this translocation in an in vitro model three types of pulmonary epithelial cells were examined. The integrity of the cell monolayer was verified by measuring the transepithelial electrical resistance (TEER) and passage of sodium fluorescein. TEER was too low in A549 cells. In these preliminary experiments, TEER values of 1007+/-300 and 348+/-62 Omega cm2 were reached for the Calu-3 cell line, using permeable membranes of 0.4 and 3 microm pore size, respectively. Growing primary rat type II pneumocytes on 0.4 microm pores, a TEER value of 241+/-90 Omega cm2 was reached on day 5; on 3 microm pores, no acceptable high TEER value was obtained. Translocation studies were done using 46 nm fluorescent polystyrene particles. When incubating polystyrene particles on membranes without a cellular monolayer, significant translocation was only observed using 3 microm pores: 67.5% and 52.7% for carboxyl- and amine-modified particles, respectively. Only the Calu-3 cell line was used in an initial experiment to investigate the translocation: on 0.4 microm pores no translocation was observed, on 3 microm pores approximately 6% translocation was observed both for carboxyl- and amine-modified particles.