The percentage of TNF alpha- and beta-positive cells was analyzed in hypertrophic scar (N = 13), normotrophic scar (N = 7), and normal skin (N = 6) biopsies using monoclonal antibodies and immunoperoxidase staining of cryostat tissue sections. Samples were first characterized for infiltrating cells. In hypertrophic samples there was a significant increase in activated infiltrating cells, capable of producing TNF beta and IL-1 beta. In contrast, the percentage of TNF alpha-positive cells was significantly lower than that detected in normotrophic scars. In fact, in hypertrophic scar samples a positive staining with anti-TNF alpha mAb was restricted to 8% of tissue-infiltrating cells compared to 35.4% of the cells present in normotrophic scars; 12% of infiltrating cells were stained in normal skin sections. These results suggest that TNF alpha may be important for normal wound healing and that hypertrophic scarring might be partially a consequence of a low amount of TNF alpha.