A microprocessor-controlled system has been developed to scan tissue culture dishes automatically in order to detect and put scribe marks at the location of fluorescently-labeled cell colonies. Using fluorescein as a,label, fluorescence as low as 25% above intrinsic cell background fluorescence can be detected. Small artifacts of selectable size (e.g. 0.2mm) are ignored by the instrument. The fluorescence signal level threshold can be set (e.g. 70% above intrinsic cell background) to within an accuracy of approximately 15% of intrinsic cell background *fluorescence and the system will reliably find colonies with a signal exceeding that level. With the above typical values, the number of false positives is typically less than 5 per dish and the time to scan an 80mm diameter tissue culture dish is two to four minutes. This time could likely be reduced somewhat in future versions by redesign of the optical path using a perpendicular rather than oblique laser beam angle of incidence with the moving dish eliminating the requirement for two half scans.
