Monocytes accumulate in the epidermis and along the dermo-epidermal junction in several different inflammatory skin diseases. To determine whether human epidermal keratinocytes elaborate a specific chemotaxin responsible for the accumulation of monocytes at these anatomic sites, monocyte chemotactic activity in conditioned 16-h cultured keratinocyte supernatants were assayed using human peripheral blood monocytes as the target cell. Dilutional analysis revealed directed monocyte migration in IFN-gamma-treated (100 U/ml) keratinocyte supernatants (80% maximal FMLP response) which was 10-fold more than IFN-gamma itself or untreated keratinocyte activity alone. Gel filtration chromatography revealed that this activity eluted just ahead of a 12.5-kDa molecular mass marker. Blocking studies demonstrated that a rabbit polyclonal antibody to monocyte chemotaxis and activating factor (MCAF) inhibited all monocyte chemotaxis by greater than 80%. Keratinocytes were metabolically labeled with 35S-cysteine/methionine, and after 16 h incubation the supernatants immunoprecipitated with the same anti-MCAF antibody. MCAF was detected as a protein doublet of 12 and 9 kDa only in IFN-gamma-treated (100 U/ml) keratinocyte supernatants. Incubation with IFN-gamma and TNF-alpha (250 U/ml) in combination resulted in increased production of MCAF protein. By Northern blot analysis, MCAF mRNA was constitutively expressed in keratinocytes and upregulated only in the presence of IFN-gamma. TNF-alpha, IL-1 beta, transforming growth factor-beta and phorbol esters had no positive or negative influence on MCAF mRNA. These studies demonstrate that biologically active MCAF is elaborated by human epidermal keratinocytes upon activation by IFN-gamma, a cytokine also required for the induction of adherence between monocytes and keratinocytes. Keratinocyte-derived MCAF is likely to be important in the regulation of cutaneous monocyte trafficking and may also be responsible for the recruitment of Langerhans cells and dermal dendrocytes, which share many phenotypic features with monocytes/macrophages, to their anatomic locations in skin.