Analysis of seeds harvested from field-grown soybeans demonstrated that simulated acidic rainfalls from two experimental protocols can significantly decrease total protein contents of soybeans. Statistically significant differences in protein content per seed mass were obtained in three of four experiments at the field site. Mean protein contents of control plants for the three experiments reported herein were 33-0, 39-3 and 43-8% and were comparable to protein yields obtained by commercial growers. When results were expressed as protein content per seed mass the changes were independent of reduction in seed mass per plant due to increased rainfall acidity. When results were expressed as protein contents per plant, decreases in protein of 20% were common in plants exposed to rainfalls similar to ambient (pH 4-1) compared with experimental controls (pH 5-6). (Copyright (c) 1984 The New Phytologist.)