Two forms of phosphoribulokinase from the alga, Scenedesmus obliquus, have been purified to homogeneity by DEAE-cellulose, Ultrogel AcA34 and hydroxyapatite chromatography. An active form of the enzyme is a dimer of identical 42,000-Mr subunits. A latent form of phosphoribulokinase, requiring incubation with dithiothreitol for activity, is of Mr 470,000 and apparent subunit composition X8Y4. The subunits X and Y are of Mr 39,000 and 42,000 respectively. The latent form of phosphoribulokinase is lost during DEAE-cellulose chromatography but this is prevented by NAD. Depolymerisation of the latent phosphoribulokinase to give the low-Mr form of the enzyme accompanied its activation by dithiothreitol. An algal protein with all the properties of thioredoxin stimulates activation of the latent phosphoribulokinase when incubated with low concentrations of dithiothreitol. The latent form of phosphoribulokinase predominates in the heterotrophically grown algae whilst under photoheterotrophic conditions equal amounts of both enzyme forms are present in algal extracts. This is consistent with the suggestion that light activation of phosphoribulokinase in vivo is also due to depolymerisation of the large-Mr latent form of the enzyme.