A gene encoding the alpha'-subunit of beta-conglycinin, a seed storage protein of soybean (Glycine max), was transformed into petunia cells on a disarmed Ti-plasmid of Agrobacterium tumefaciens, and plants were regenerated. Transcripts of the introduced gene accumulated in immature embryos but not in leaves of the transformed plants. Soybean protein was first detected immunologically in proteins extracted from embryos at 10 days post pollination (d.p.p.), concurrent with the accumulation of subunits of the major petunia seed proteins. Between 10 and 16 d.p.p. the primary soybean protein detected had an apparent mol. wt. of 55 kd. The 76-kd alpha'-subunit and several smaller polypeptides accumulated between 16 and 24 d.p.p., when seeds had matured. Polypeptides <76 kd probably resulted from specific proteolytic cleavage of the alpha'-subunit. The alpha'-subunit and the smaller polypeptides assembled into multimeric proteins with sedimentation coefficients of 7-9S, similar to the sedimentation coefficients of beta-conglycinins isolated from soybean seeds. This transformation and expression system should be ideally suited for testing gene mutations to alter the amino acid composition of these seed storage proteins.