A particle bombardment technique was used for gene transfer to human peripheral blood mononuclear cells, and murine splenocytes, thymocytes and peritoneal macrophages in primary culture. Significant expression of a luciferase marker gene was observed in these cell types within 8 h of gene transfer. Luciferase expression was readily detected in peritoneal macrophages 4 h after culture initiation and transfection. Same day determinations of transgene activity in fresh human peripheral blood mononuclear cell samples were feasible. Promoter preference and ballistic parameters were examined to optimize transgene expression. Up to 6% of bombarded human T lymphocytes expressed transgenic beta-galactosidase activity. These results demonstrate that particle bombardment is an effective means for gene transfer and provides an attractive approach for rapid, quantitative analysis of transgene expression in various leukocyte primary culture systems.