Based on the hypothesis that arsenic exposure results in toxicity and mitogenecity, this study examined the dose-response of arsenic in established human cell lines of keratinocytes (HaCaT), melanocytes (1675), dendritic cells (THP-1/A23187), dermal fibroblasts (CRL1904), microvascular endothelial cells (HMEC), monocytes (THP-1), and T cells ( Jurkat). Cytotoxicity was determined by incubating THP-1, THP-1+ A23187 and JKT cells in RPMI 1640, 1675 in Vitacell, HMEC in EBM, and dermal fibroblasts and HaCaT in DMEM with 10% fetal bovine serum, 1% streptomycin and penicillin for 72 hrs in 96-well microtiter plates, at 37degreesC in a 5% CO2 incubator with different concentrations of arsenic using fluorescein diacetate (FDA). Cell proliferation in 96-well plates was determined in cultured cells starved by prior incubation for 24 hrs in 1% FBS and exposed for 72 hours, using the 96 cell titer proliferation solution (Promega) assay. Cytotoxicity assays yielded LD50s of 9 mug/mL for HaCaT, 1.5 mug/mL for CRL 1675, 1.5 mug/mL for dendritic cells, 37 mug/mL for dermal fibroblasts, 0.48 mug/mL for HMEC, 50 mug/mL for THP-1 cells and 50 mug/mL for JKT-T cells. The peak proliferation was observed at 6 mug/mL for HaCaT and THP-1 cells, 0.19 mug/mL for CRL 1675, dendritic cells, and HMEC, and 1.5 mug/mL for dermal fibroblasts and Jurkat T cells. These results show that arsenic is toxic at high doses to keratinocytes, fibroblasts, monocytes and T cells, and toxic at lower doses to melanocytes, microvascular endothelial cells and dendritic cells. Proliferation studies showed sub-lethal doses of arsenic to be mitogenic.