Large changes in indole-3-acetic acid (IAA) levels occur during growth of Lemna gibba G-3 in sterile culture. The levels of IAA were measured in plants during a 45 day growth cycle using HPLC and isotope dilution analysis followed by selected ion current monitoring GC-MS analysis with 13C6-IAA as the internal standard. Even though the rate of plant growth remained constant over the entire growth period, IAA levels ranged from a high of 222 to a low of 6 nanograms per gram fresh weight. A Lemna mutant (jsR,) which has a giant phenotype was obtained by regeneration from primary callus cultures. Microspectrofluorometry of diamidino-2-phenylindole stained cells showed that jsR, has the same amount of DNA per nucleus as the parent line (PL). All jsR, cell types measured are about 1.5 times larger than in PL. The endogenous levels of IAA per gram fresh weight were higher in jsR, at several stages of the plant culture cycle as compared to PL. This difference ranged from 1.2 to over 100 times as much. While PL showed only one high peak at day 9, jsR, had IAA levels of 480 and 680 nanograms per gram fresh weight at days 9 and 45, respectively. Throughout the midculture stage of the growth cycle (20-28 days) both jsR, and PL had IAA levels in the range of 9 to 14 nanograms per gram fresh weight. In contrast to PL, at day 45, jsR, had no detectable ester or amide conjugates of IAA. These changes in IAA levels were determined in sterile plant cultures and thus cannot be attributed to bacterial or fungal activity.