We have initiated a research programme on the factors that determine the success of rooting of microcuttings in vitro. This research is carried out in Malus and encompasses physiological, hormonal and molecular regulation. Here we report on the role of indolebutyric acid (IBA). The effect of IBA depends upon the length of application and upon the concentration. It is concluded that it is not the concentration but the dose of the hormone taken up which is decisive for the number of roots formed. IBA is rapidly conjugated in the tissue, probably to IBA-aspartate. In contrast to various reports in literature, we observed no conversion of IBA into indoleacetic acid (IAA), A product of photoinactivation of IBA closely co-chromatographed with IAA. In the medium, photo-inactivation of IBA occurred rapidly: at 80 pmol s ' m 5 after 32 h, 30% or 88% of the IBA had disappeared in the absence or presence of riboflavin (1 mgl '), respectively. In the dark, no photo-inactivation was observed. Promotion of rooting by darkness is possibly caused in part by a lack of photo-inactivation of IBA. On tic-plates, photo-inactivation of IBA was very rapid.