A new general method for sequencing fluorescently labelled DNA by chemical degradation has been developed. It is based on the observation that fluorescein attached via a mercaptopropyl or aminopropyl linker arm to the 5'-phosphate of an oligonucleotide is stable during the reactions commonly used in chemical cleavage procedures. DNA to be degraded is first enzymatically synthesized in vitro by annealing and extending a fluorescently labelled primer thereby introducing the fluorescent label at the 5'-end of the fragment. The newly synthesized fluorescently labelled DNA is then chemically degraded using: (a) a set of four different cleavage reactions; or (b) only one reaction comprising methylation of G-residues followed by a partial cleavage with piperidine in the presence of sodium chloride. The fluorescent degradation products are loaded on either four lanes or one lane of the gel, respectively, and the emitted fluorescence detected online during electrophoresis. In the 'four reactions/four lanes' method 200-350 bp (base pairs) can be read from the labelled end. The 'one reaction/one lane' method, in which the nucleotide sequence is determined by measuring different signal intensities following the rule G greater than A greater than C greater than T, currently yields around 100-200 bp of sequence per sample.