Break-Free CLP is a commercial petroleum-based liquid used for cleaning, lubricating, and protecting firearms that is used in the United States by military personnel, police, and individual gun owners for maintaining a wide variety of firearms. According to its material safety data sheet (MSDS), Break-Free CLP is predominately polyalphaolefin oil but also contains dibasic ester and isoparaffinic hydrocarbons; all of these ingredients are known to induce skin irritation in laboratory animals. Studies completed in our labs found that repeated topical application of Break-Free CLP to the backs of CD-1 mice produced evidence of systemic effects. Studies were conducted to characterize the dermal penetration of Break-Free CLP in mouse, rat, and pig skin to provide insight on possible factors or causes of skin irritation and systemic effects observed in previous studies. Mouse skin was 37 times more permeable to Break-Free CLP than pig skin and 6 times more permeable than rat skin. Flux measurements from static diffusion cells showed an inverse correlation with mouse, rat, and pig skin thickness. The concentration of Break-Free CLP in mouse skin was 4.5 times higher than the amount found in rat skin and about 17 times higher than the amount absorbed by pig skin. These results support the idea that Break-Free CLP causes skin irritation and systemic effects in the mouse by both penetrating through and accumulating in the skin. The findings for rat and pig skin are probably most representative of Break-Free CLP flux into and through unprotected human skin and suggest that dermal toxicity studies in CD-1 mice overestimate the risk to humans. These results, nevertheless, suggest that persons handling or using Break-Free CLP should protect the skin from possible exposure.