Sodium hypochlorite, the most widely used antimicrobial active chlorine compound in chemical disinfection, is little used as an antiseptic in clinical practice. This study aimed to assess the capacity of hypochlorite to alter human dermal fibroblast growth in vitro in relation to the concentration and exposure time. Effects of decreasing concentrations of hypochlorite (0.5%–0.00025%) on fibroblast adherence capacity and proliferation, according to varying exposure times and fetal calf serum (FCS) concentrations were investigated combining XTT assay, which provides cytochemical quantification of metabolically-active cell number, and total cell protein content, an indirect method for assessing substrate-adhered cell number. Initial cytotoxicity was produced at 0.0075% hypochlorite within contact time of two hours, provoking concentration-dependent cell detachment. From 0.1% upwards, NaOCl exerted a profound cytotoxic effect on fibroblasts. At later stages (4 h) and concentrations ≥0.01% hypochlorite produced dose-dependent mitochondrial dysfunction: cell survival progressively diminished from 71% to 10%. Cytotoxic effects were not significantly affected by exposure-time periods, probably because maximum chlorine is released within the first four hours. Hypochlorite concentrations from 0.005% to 0.00025% were found to have no inhibitory effects on cell growth; in fact, they appear to exhibit the opposite effect. Increments in protein content found after 24 h exposure ranged from 30% to 120% above control values. Hypochlorite is highly cytotoxic for fibroblasts at concentrations ≥0.01% provoking concentration-dependent loss of cell adherence capacity and mitochondrial dysfunction. In contrast, a mitogenic effect was observed with concentrations ≤0.005% which supports NaOCl as a source growth-promoting activity in cultured human fibroblasts. Hypochlorite proved to be a highly reactive molecule which inhibits or stimulates cell division according to the concentration.