The formation of complexes of gibberellic acid (GA3) and proteins under in vitro conditions was studied. It was shown that labelled GA3 binds to soluble cytoplasmic proteins, although a considerable amount of radioactivity remains in the pellet containing nuclei and cell debris. GA3-protein complexes are excluded from Sephadex G-10 column with the void volume. They sediment in linear sucrose density gradients as three distinct peaks, having higher S values than bovine serum albumin, used as a marker. Soluble GA3-protein complexes can be separated into four zones of radioactivity upon ion exchange chromatography on DEAE-Sephadex A-50 column, each of them eluting with a different KCl concentration. Agarose gel electrophoresis of GA3-protein complexes reveals two zones of radioactivity at the anodic part of the electrophoretogram. After extraction of the complex with ethanol, more than 90% of radioactivity is found in the ethanolic phase, which indicates that the binding is not covalent. GA4+7 and GA13 decrease the binding of GA3 to cytoplasmic proteins for 30%, suggesting that some common binding sites exist at the same binding proteins.