An enzymatically optimized, miniaturized (20 microl) fluorimetric assay of galactose-1-phosphate-uridyltransferase using dried blood spots for newborn screening is presented. The Beutler reaction principle has been adapted to the microtiter plate technology and acetone/methanol was used for complete deproteinization. A special ultramicro multiwell screening plate resistant to organic solvents has been developed and employed. The assay is simple, sensitive and inexpensive, due to small reagent volumes and the low prices of ultramicro screening plates. The reaction is linear with galactose-1-phosphate-uridyltransferase activity up to 120 min of incubation time. It shows low imprecision and good correlation to a quantitative validation test. For standardization the use of plate means or medians of activity or fluorescence values is proposed. Individual blank measurement prevents false negative assessments.