The effects of the low power laser irradiation (Space Mix 5 Mid Laser) on the 3H-proline incorporation in the collagenic proteins produced by two lines of normal human fibroblasts in vitro were studied. The 3H-thymidine incorporation in cultures of control and irradiated fibroblasts of the same lines was also evaluated. The obtained results show that in the experimental conditions considered, laser irradiation may have a positive effect on the production of collagen by the fibroblasts in vitro. This effect is dependent on the time of exposure, the frequency of infra-red impulses and the age of subjects from which fibroblasts were obtained. The laser irradiation in the same experimental conditions does not affect the incorporation of 3H-thymidine in the fibroblasts in vitro. Therefore, the observed increase of collagen production is not dependent on an increase of cellular population.