ABSTRACTSingle cell suspensions of epidermal cells from guinea pigs were analyzed histochemically for the presence of the following enzymes: 5′-adenosine triphosphatase, nonspecific esterase, specific esterase, myeloperoxidase and leukocyte alkaline phosphatase. A population of cells was positive for nonspecific esterase, leukocyte alkaline phosphatase, and 5′-adenosine triphosphatase. This population was identified as Langerhans' cells because the number of cells stained for these enzymes paralleled the number of Langerhans' cells in the suspension. These same enzymes were shown to be present in guinea pig leukocytes of the mononuclear–phagocytic series, suggesting that they and Langerhans' cells may have a precursor in common.