Most quantitative in vitro approaches to determine irritancy have examined the potential of compounds to decrease biological functions or inhibit growth of cells. Irritants, however, are known to generally have the opposite effect in vivo, i.e. to stimulate cell division. This property has not been directly studied in vitro. We examined the ability of sublethal concentrations of sodium lauryl sulfate (SLS) to stimulate cultured keratinocyte and fibroblast proliferation in vitro. The growth of keratinocytes, without added growth factors, continuously exposed to SLS for 4 days was stimulated approximately 89% compared to control. Keratinocytes exposed to SLS for 1 or 18 h were stimulated 36 and 12%, respectively, over the next 4 days of growth. Subconfluent fibroblasts were also stimulated approximately 38%. Confluent fibroblasts were stimulated 40%. All stimulations were maximal between 10––8 and 10––5M added SLS. Media conditioned by keratinocytes exposed to 10––8M 10––5M inhibited cell growth. We conclude that sublethal doses of SLS can stimulate the growth of cultured keratinocytes and fibroblasts. The stimulation of growth seen may be related to the stimulation observed in in