The effects of sulfur deficiency on the complement of proteins laid down in developing seeds of soybean (Glycine max L. Merr) have been examined. Sulfur deficiency caused a 40% decrease in the level of glycinins and a contrasting elevation in the level of beta-conglycinins. The subunit composition of these proteins was also affected. There was in particular a 3-fold increase in the beta-subunit of beta-conglycinins in the sulfur-deficient seeds, and this accumulated largely as the B(0)-isomer of beta-conglycinins, a protein which while virtually devoid of methionine and cysteine retains the physical properties of a normal 7S storage protein. These data demonstrate that a high degree of selectivity can be exerted by environmental stress over the accumulation of proteins in developing seeds.