High-pressure sodium (HPS) lamps are popular for plant lighting because of their high energy conversion efficiencies. Yet their spectrum has very little blue light (BL), which may cause undesirable morphological responses. To study this, McCall' soybean [Glycine max (L.) Merr.] plants were grown for 28 d in growth chambers using HPS lamps, with or without supplemental light from blue phosphor fluorescent lamps. Total photosynthetic photon flux (PPF) levels (including blue fluorescent) were kept near 300 or 500 micromoles m-2 s-1. Blue fluorescent levels ranged from 7 to 20 micromoles m-2 s-1, providing from 6 to 18 micromoles m-2 s-1 of supplemental BL (400-500 nm). Stem and internode lengths were longest under 300 micromoles m-2 s-1 HPS lighting and became progressively shorter with increasing supplemental BL until a total of approximately 30 micromoles m-2 s-1 of BL (from HPS and BL supplement) was present in the spectrum. Beyond this, extra BL had no effect. Two other lamps rich in BL, metal halide (Optimarc) and fluorescent (Vita-Lite), also produced plants with short stems, as did HPS lighting maintained at 500 micromoles m-2 s-1. Results suggest that use of high-pressure sodium or other blue-deficient sources for lighting at low to moderate photosynthetic photon flux levels may cause abnormal stem elongation, but this can be prevented by adding a small amount of supplemental blue light.