Nanoparticles are available in a wide range of products, like pharmaceutical product and cosmetics and in the last decade it has been an increasing use of silica nanoparticles. Nanoparticles are defined as particles with a diameter range from 1 to 100 nm in at least one dimension. They have unique physicochemical properties compared to larger-sized particles of similar chemical composition. This is mostly explained by their small size, larger surface area and surface reactivity. Since there still is some lack of knowledge on their effects on human health and environment, further research on their chemical properties and potential toxicity is required. Previously, silica nanoparticles (SiNPSSs) of 10 and 50 nm size have been shown to give inflammatory (cytokine) responses in epithelial lung cells. In this master thesis we focus on the ability of different sized silica particles to induce cytokine response in, both, macrophages (differentiated THP-1 cells) and in epithelial cells (BEAS-2B). The aims in this project include deciding the best exposure conditions in which the cells respond to the Si-particles by cytokine release, without giving too much cytotoxicity. This is then followed by comparing different sized amorphous silica particles: 10, 12 and 500 nm, for cytotoxicity and cytokine release. Finally, the effects of one of the nanoparticles are analyzed, with regard to transfer of conditioned medium from SiNPSS- exposed differentiated THP-1 cells to another cell culture. THP-1 cells cultured in RPMI-1640 with 10% FBS were differentiated to more macrophage-like cells with PMA (50 ng/ml) for 48 h in advance of experiments, while the BEAS-2B cells were cultured in LHC-9 medium. The cells were then exposed for the particles for 6 or 20 h. The cytokine release (IL-1β, TNF-α and IL-8) was measured by ELISA, while gene expression was measured by Real time PCR and the cytotoxicity was investigated by AlamarBlue assay and by release of lactate dehydrogenase (LDH). Cells exposed in medium without FBS gave increased cytotoxicity and a significantly higher release of cytokines, compared to the medium with 10 % FBS. When comparing different Si-particles, the differentiated THP-1 cells responded markedly with release of IL-1β, TNF-α and IL-8, with Si12 and Si500 as most and least potent, respectively. The gene expression of IL-1β and TNF-α due to Si10 exposure indicated a time-dependent increase. The gene expression of IL-1β did not have a significant increase before the 6th h of exposure, while the gene expression of TNF-α reached maximum after 4.5 h. However, the gene expression did not seem to be up-regulated ahead of the cytokine release. Considering, the airways consist of several cells interacting, and investigation of possible inetacting reaction between different cell cultures, were done. The transfer of conditioned medium to unexposed cell cultures showed a possibility for effect on BEAS-2B cell, while the differentiated THP-1 cell mostly seemed unaffected. The model system of differentiated THP-1 cells seems suitable for further mechanistic studies, but using different batches of a cell-culture seems influence the results. Furthermore, studied about interactions between cell cultures and cell types have to be explored more.