Arachidonic acid (AA) release and metabolism to prostaglandins (PG) in response to the complete mouse skin tumour promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and the first-stage promoter A23187 were compared in keratinocytes from normal adult human and two different strains of mouse with different sensitivities to TPA as a tumour promoter. The rate, extent and distribution of incorporated [14C]AA among the classes of phospholipids were similar in cultures of either CD-1, SSIN or human keratinocytes. Using prelabelled cultures, the amount of radiolabel released in control cultures was nearly identical for human and both strains of mice. Distinct species differences were observed, however, after TPA treatment. Cultures of human epidermal keratinocytes (HEK) released only half as much label by 6 hr compared with either strain of mouse. In addition, whereas the mouse epidermal keratinocytes (MEK) metabolized AA readily to PGE2, very little PGE2 could be detected in the human cultures. The extent of variability between individual human samples (n = 11) in response to 0.1 μg TPA/ml ranged from a 20% increase to a 200% increase in release of label, with a mean increase of 50%, whereas murine cells produced a mean increase of 400%. When MEK and HEK were stimulated with 10-6 and 10-5 m-A23187, an increase in the release of arachidonate by 200 and 400%, respectively, was observed for both species. Under these conditions of equal release, equivalent amounts of PGE2 were produced. To compare further the ability of mouse and human cells to metabolize exogenous AA to PGE2, freshly isolated, as well as cultured, cells from each species were incubated with [14C]arachidonate. Under both conditions, HEK have approximately the same ability as MEK to metabolize AA to PGE2 (approx. 2% of the arachidonate for both species). The reduced ability of HEK, compared with MEK, to produce PGE2 is specific to TPA and is due primarily to insufficient substrate, that is, low levels of arachidonic acid release.