Blue light is known to be antimicrobial, but its effect on normal cutaneous and subcutaneous cells remains unclear. Therefore, we studied the effect of 470-nm light on the viability of adult and neonatal human dermal fibroblasts, Jurkat T-cells, and THP-1 monocytes in vitro. Each culture was irradiated with 0, 3, 55, or 110 J/cm of 470-nm light and subjected to trypan blue assay to ascertain viability. Further, MTT, neutral red, and fluorescence assays of fibroblasts were performed, and cell morphology visualized using bright field and fluorescence microscopy. At each dose and in each of the four cell lines, there was no significant difference in cell concentration between irradiated and non-irradiated cultures, even though irradiation with 55 J/cm or 110 J/cm slightly decreased cell count. Light microscopy showed progressive morphological changes in the fibroblasts as energy fluence increased from 55 to 110 J/cm. Irradiation at 3 J/cm produced a slight but non-significant increase in the viability of Jurkat T-cells and THP-1 monocytes. In contrast, at 110 J/cm radiant exposure, irradiation slightly decreased the viability of all four cells. While 3 J/cm appears stimulatory, our finding that 110 J/cm produces a slight decrease in viability and engenders morphological changes in fibroblasts, suggesting that such high doses should be avoided in blue light treatments.