Human forearm fibroblasts were irradiated, in vitro, using a Microlaser Medical Pen Unit (Johnson and Johnson) with a wavelength of 860 nm, and an average power output of 50 mW. The irradiation distance and times were adjusted to give energy densities of 2 J/cm² and 16 J/cm² respectively. The control unit was adjusted to give a continuous waveform. Fibroblast proliferation and succinic dehydrogenase activity were assessed every 24 hrs over a five day period. The results obtained suggest a relationship between fibroblast proliferation and succinic dehydrogenase activity. At the lower energy density (2 J/cm²), succinic dehydrogenase levels rise with fibroblast proliferation. However, at an energy density of 16 J/cm², the succinic dehydrogenase levels and fibroblast proliferation appear to be inhibited. These findings agree with previously reported data.