Phytoene synthase activity in tomato chloroplasts is membrane-associated, requiring treatment with high ionic strength buffer or mild non-ionic detergent for solubilisation. Using a combination of ammonium sulphate precipitation, cation and anion exchange, dye-ligand and hydrophobic interaction chromatography, phytoene synthase has been purified 600-fold from tomato (Lycopersion esculentum Mill.) chloroplasts. The native molecular mass of the enzyme was 43 kDa. with an isoelectric point of 4.6. Although phytoene synthase was functional in a monomeric state, under optimal native conditions it was associated with a large (at least 200 kDa) protein complex which contained other terpenoid enzymes such as isopentenyl diphosphate isomerase and geranylgeranyl diphosphate (GGPP) synthase. Both Mn2+ and ATP, in combination, were essential for catalytic activity; their effect was stochiometric from 0.5 to 2 mM, with Km values for Mn2+, ATP and the substrate GGPP of 0.4 mM, 2.0 mM and 5 microM, respectively. The detergents Tween 60 and Triton X-100 (0.1 w/v) stimulated (5-fold) enzyme activity, but lipids (crude chloroplast lipids and phospholipids) had no such effect and could not compensate for the absence of detergent. A number of metabolites with possible regulatory effects were investigated, including beta-carotene, which reduced enzyme activity in vitro some 2-fold. A comparison of phytoene synthase activity from partially purified chloroplast and chromoplast preparations indicated biochemical differences.