Time-gated fluorescence spectroscopy was performed on the tumour-localizing fraction (TLF) of haematoporphyrin derivative (HPD) incorporated into cells. Three different cell lines were incubated with 20 and 5 micrograms ml-1 of TLF for various time periods; they were then washed and resuspended in buffer. Fluorescence decay measurements and time-integrated and time-gated spectra were then obtained from the cell suspensions. Similar experiments were repeated using HPD containing 60% of the active material. The experimental results show a modification of the emission spectra for both drugs depending on the incubation time; this modification is more significant for the TLF. In particular, the emission peak observed in aqueous solution at 615 nm is shifted to 630 nm as a consequence of incorporation into cells, and the gated spectra indicate that the fluorescence emission is mainly related to monomers and unfolded polymeric chains. The ratio between the intensities of the two peaks depends on the relative amount of the TLF; the peak at 615 nm is more pronounced for HPD. The results obtained seem to indicate that both the composition of the drug and the metabolic properties of the biological environment strongly influence the uptake process and the fluorescence behaviour of the incorporated sensitizer.