Magnolia Flower is a crude drug used for the treatment of headaches, toothaches, and nasal congestion. Here, we focused on Magnolia kobus , one of the botanical origins of Magnolia Flower, and collected the flower parts at different growth stages to compare chemical compositions and investigate potential inhibitory activities against interleukin-2 (IL-2) production in murine splenic T cells. After determining the structures, we examined the inhibitory effects of the constituents of the bud, the medicinal part of the crude drug, against IL-2 production. We first extracted the flower parts of M. kobus from the bud to fallen bloom stages and analysed the chemical compositions to identify the constituents characteristic to the buds. We found that the inhibitory activity of the buds against IL-2 production was more potent than that of the blooms. We isolated two known compounds, tiliroside ( 1 ) and syringin ( 2 ), characteristic to the buds from the methanol (MeOH) extract of Magnolia Flower. Moreover, we examined the inhibitory activities of both compounds against IL-2 production and found that tiliroside ( 1 ) but not syringin ( 2 ), showed strong inhibitory activity against IL-2 production and inhibited its mRNA expression. Thus, our strategy to examine the relationship between chemical compositions and biological activities during plant maturation could not only contribute to the scientific evaluation of medicinal parts of crude drugs but also assist in identifying biologically active constituents that have not yet been reported.