The marked difference in auxin sensitivity between plant roots and shoots was studied in terms of auxin-induced stimulation of membrane P-type ATPase. The results suggest the existence in rice seedlings of, at least, two isoforms of soluble proteinous factors (SPF), SPF(I) and SPF(II), which are involved in the stimulatory action of auxin on the enzyme. It also is indicated that SPF(I) which mediates the auxin effect in low hormone concentration range (10(-10)-10(-7) M IAA) is the dominant isoform in roots, whereas SPF(II) which does in high hormone concentration range (10(-7)-10(-4) M IAA) is that in shoots.