Methods for high efficiency gene transfer into primary cells of various lineages and disease states are desirable, as they remove the uncertainties associated with using transformed cell lines. Adenoviruses have evolved to deliver their genes into cells with high efficiency and in recent years have been exploited as a gene transduction system. Prior to the discovery of adenoviruses, efficient expression of transgenes was only possible by cloning stably transfected cells; this was limited to cell lines and was not an option for primary cells. Here we describe a method of transgene expression, which enables previously untransfectable cells, such as primary myeloid cells or diseased synovium, to express protein at extremely high levels with nearly 100% of cells expressing the transgene. This allows us to examine the effect of target genes on signaling pathways in primary cells without the need for cell sorting or the simultaneous transfection of reporter genes. This is very important in studies of tissues such as rheumatoid synovium where sorting of cells will damage the biological value of the system.