A simple system has been designed enabling ultrasensitive on-line detection of fluorescently labelled macromolecules, e.g. nucleic acids, proteins and peptides during electrophoretic separations in gels. An important application is the automated DNA sequence determination without radioactivity. Drying of gels, film exposure and handling are not necessary. A sulphydryl containing M13 sequencing primer has been synthesised and end-labelled in a reaction with fluorescein iodoacetamide. This is then used in the dideoxy reactions. In particular no moving parts or complicated software are required for data collection and analysis. Compared to our first automated device detection sensitivity has been improved by a factor of thirty to about 3 X 10(-18) mol per band. The resolution has increased to about 400 bases in 5 hours, with the possibility to read up to about 500 bases when they are properly labelled. Gels shorter than 20 cm may be used for resolution of about 300 bases. The single gel system may be upgraded for simultaneous running and reading of six or ten sequencing samples.