In vitro exposure to aerosols at the air-liquid interface (ALI) preserves the physical and chemical characteristics of aerosol particles. Although frequently described as being a more physiologic exposure method ALI exposure has not been directly compared with conventional in vitro exposures where the particles are suspended in medium. We exposed immortalized human bronchial epithelial cells (16HBE140) to aerosolized diesel exhaust particles at the ALI and to suspensions of collected particles. The response of the cells was determined from measurements of the cell viability and interleukin-8 (IL-8) secretion. The deposited size distribution at the cell surface was measured with transmission electron microscopy to obtain a dose for the ALI exposure. Although exposure by either method caused a slight decrease in cell viability and induced IL-8 secretion, the response to ALI exposure occurred at doses several orders of magnitude lower than exposure to particles in suspension. The most likely sources for the different dose response are the artifacts introduced during the collection and resuspension of particles for conventional suspension exposures. The number concentration of particles deposited at the ALI is similar to the modeled deposition in the tracheal-bronchial region in a human lung, but the ALI size distribution is skewed toward particles larger than those deposited in the lung.