In an attempt to produce chloroplast extracts containing ribulose 1,5-bisphosphate (RuBP) carboxylase in its fully activated state, MgCl 2 and NaHCO 3 were included in the medium used to osmotically shock chloroplasts. Extracts prepared in this manner contained lower levels of the enzyme than those prepared in the absence of MgCl 2 and NaHCO 3. The difference in enzyme levels was found to be attributable to an association in the presence of Mg 2+, between RuBP carboxylase and the thylakoids removed from the extract during its preparation. Some monovalent cations caused a similar association, although to a lesser extent. The trivalent cation Tris(ethylenediamine) cobalt(III) was more effective in causing this association, but was highly inhibitory to the enzyme. The results suggest that the attraction between thylakoids and RuBP carboxylase in the presence of certain ions is likely to be electrical in nature. The results are discussed in terms of the media used to isolate RuBP carboxylase.