A series of constructs containing the developmentally regulated soybean lectin gene (Le1) were used to transform tobacco plants in order to assess developmental and quantitative regulation conferred by flanking sequences. The largest of the lectin constructs contained approximately 3,000 base pairs (bp) of Le1 5 flanking region and 1,500 bp of the 3 flanking region. The smallest construct contained no 5 flanking region and 194 bp of the 3 flanking region. ELISA assays of lectin in individual tobacco seeds and Southern blot analyses confirmed that most constructs were inherited as unique insertion events. Maximal expression of Le1 required more than 338 bp of 5 sequence, indicating that far upstream factors are involved in quantitative control of lectin expression. Lectin expression declined more than 80% between deletions with 1,700 versus 338 bp of 5 flanking sequence. In contrast, developmental control of lectin expression was maintained by Le1 inserts with only 190 bp of 5 sequence. The lectin promoter offers a potential means to target high levels of gene expression to the developing seeds of soybean or other dicotyledonous plants.