In the wheat cylinder bioassay technique as previously used here 5 sections have been enclosed in a 2 × ⅜38 in, assay tube together with 0.5 ml. of the test solution. A method developed for estimating the amount of carbon dioxide which accumulates in these tubes through the respiration of the enclosed sections has shown that the level can rise to 20 per cent. after 24 hrs. at 25°C. In the presence of a 100 p.p.m. IAA(6×10-4M.) test solution, growth of 5 enclosed sections is depressed from 8 hrs. onwardas and they eventually shrink, releasing their accumulated IAA back into the solution. The growth of sections under various gas mixtures of carbon dioxide in air has also been followed and these experiments show that section length is reduced approximately lineraly with respect to increasing carbon dioxide concentration up to 20 per cent. in air, both in the presence and absence of a 100 p.p.m. IAA solution. The slope of the fitted regression line, however, is much steeper when the test solution contains IAA—i.e. there is a large interaction. In the presence of IAA, growth-time data show that a reduction in the growth rate, as compared with that in normal air, can be detected after only 4 hrs, at the highest carbon dioxide concentration. In the absence of IAA, high concentrations of carbon dioxide accelerate growth during the first 8 hrs. of the assay but depress it later. The mechanism of action of this interaction is unknown but it is not shown at very high concentrations of IAA, e.g. 1,000 p.p.m. (6×10-3M.).