Transient transfection is one of the most commonly used techniques to study gene regulation. Different reporter genes have been used to quantitate the promoter activity. CAT (chloramphenicol acetyltransferase) is the reporter gene most often used in these experiments (1). For increased sensitivity, growth hormone and luciferase have also been used as reporter genes (2, 3). For a rapid analysis of gene expression, we describe here a fast and convenient transfection protocol for growth hormone reporter gene. Because growth hormone is secreted, we have scaled down the number of transfected cells so that cell cultures on 96-well microtiter plates are adequate for routine work. Using this transfection protocol, amounts of DNA, chemicals and nutrients are similarly reduced. The protocol is convenient for multi-variable studies and should facilitate screening for factors and treatments that can result in synergistic stimulation or repression of gene activity. As shown in Fig. 1, results from the mini-transfections are compatible with those obtained by large scale CAT assay or RNase protection technique.