Two low energy lasers, helium-neon (He-Ne, continuous wave, 632.8 nm), and gallium-arsenide (Ga-As, pulse, 904 nm), were tested for their effects on connective tissue metabolism by human skin fibroblasts in culture. The cells were subjected to laser treatment at various energy fluence, and the treatment schedule included one or two daily exposures on several consecutive days. The results indicated that procollagen production was enhanced by both lasers approximately 4-fold in average. The highest enhancement of 36-fold was noted in cultures which initially synthesized procollagen at a relatively low level, while a lesser effect was achieved in cultures which already actively synthesized procollagen. The lasers tested had no effect on the activities of collagenase and gelatinase, two proteolytic enzymes controlling the degradation of collagen, in fibroblast cultures. The increased procollagen production by either one of the lasers could not be explained on the basis of enhanced cell proliferation. Also, the viability and the ultrastructural organization of the fibroblasts remained unaltered by these lasers. Thus, the results suggest that low energy lasers, with appropriate treatment schedule, might provide a safe and efficient modality for enhancement of collagen deposition in chronic necrotic processes, as in leg ulcers and pressure sores.