Leukemic cell lines such as Mono Mac 6 provide an excellent model for studying changes in gene expression during induction of cell differentiation. Mono Mac 6 cells can be induced to differentiate from their immature state to cells resembling morphologically and functionally mature monocytes and macrophages by various stimuli such as calcitriol and transforming growth factor-beta. During differentiation, the expression of differentiation markers such as the cell surface antigen CD14 or other differentiation-related genes such as 5-lipoxygenase are strongly increased. Thus, this cell line constitutes an excellent model system to study the regulation of gene expression by inducers of cell differentiation. However, myeloid cell lines are often refractory to transfection by calcium phosphate or DEAE dextran so that reporter gene assays are difficult to perform. We have established a transient transfection protocol for Mono Mac 6 cells using electroporation, a 5-lipoxygenase promoter luciferase reporter gene construct, and the secreted alkaline phosphatase as an internal standard.