The cell line KG-1 has been used as an in vitro model for human dendritic cell (DC) differentiation. We have investigated the response of KG-1 cells to stimulation with a number of factors known to induce differentiation and/or maturation of DCs in vitro. KG-1 cells showed no differentiation in response to LPS, CpG oligodeoxynucleotide or CD40 ligation. Culture in the presence of TNF-alpha induced some differentiation, but only treatment with PMA and ionomycin (with or without prior culture in GM-CSF and IL-4) induced morphological and phenotypic changes consistent with DC-like maturation, and even these maximally differentiated KG-1 cells showed lower levels of surface marker expression, macromolecular endocytosis, and ability to stimulate in allogeneic MLR compared with in vitro monocyte-derived DCs. Our data show that KG-1 cells differentiate in vitro into cells with DC-like functional characteristics under the influence of strong inducers of cellular activation, but lack the potency of mature DCs in key aspects of professional antigen presenting cells.