Scattered clinical reports suggest that low power (LP) laser irradiation may induce a biostimulation of cell growth and/or metabolism, especially relating to healing processes. On the other hand, few basic science, in-depth reports relating to such effects have appeared. Hence, a mechanism of action of LP laser irradiation on cells is unknown. A systematic evaluation has been undertaken in order to define more clearly the experimental conditions for producing biostimulation and to provide some basis for action of LP laser irradiation. A Ga-Al-As diode laser emitting in the near infrared (904 nm) was used to effectively penetrate cells at energy levels that are in the mW range. The LP laser was pulsed at 50 ns and 200 hz. Human fibroblasts growing in culture served as the experimental model. Since LP laser irradiation has been reported to stimulate collagen synthesis, we first investigated the induction of hydroxyproline formation, a collagen precursor. This biosynthetic process could be increased two-fold at a twice daily energy input of 4.5 mJ. With proline supplementation, hydroxylation increased eight-fold. At approximately the same energy level and irradiation conditions, cells also had a three-fold increased uptake of ascorbic acid, a required cofactor for hydroxylation of proline. These findings considered together with published biochemical studies of collagen suggest that higher levels of intracellular ascorbate catalyze hydroxylation of proline and, concomitantly, induce collagen formation. Other data relevant to cell morphology and viability suggest that the LP laser irradiation had no effect on cell proliferation but rather was a transient effect on intermediary metabolism manifested as changes that may be unique to collagen.