Two methods, which relate grain number to cell cycle phase in Feulgen-stained autoradiographic preparations, have been developed and compared. Both methods automate grain number estimations, one by taking integrated absorbance measurements at different wavelengths, the other by measuring absorption at a single wavelength before and after chemical removal of silver grains. With tritium-labelled tobacco mosaic virus as a probe, a quantitative analysis has been made of the binding of virus particles to plant protoplasts in different compartments of the DNA replication and partition cycle. The preliminary results indicate that the quantity of virus bound by protoplasts is related to their cell cycle phase. Whilst in this case, the methods have been used with plant cells, both techniques are equally applicable to animal cells.