Soil samples were collected from 16 medicinal plant rhizospheres in Lumphun Province. Indole acetic acid (IAA) production: The actinomycete discs (8 mm), grown on yeast malt extract agar (YM) incubated at 28 ℃ for 5 days, were inoculated into 5 ml YM broth containing 0.2% L-tryptophan and incubated at 28 ℃ with shaking at 125 rev/min for 7 days. Cultures were centrifuged at 11,000 rev/min for 15 min. One milliliter of the supernatant was mixed with 2 ml of Salkowski reagent. Appearance of a pink color indicated IAA production. Optical density (OD) was read at 530 nm using a spectrophotometer. The level of IAA produced was estimated by comparison with an IAA standard. The actinomycete discs (8 mm), grown on YM agar incubated at 28 ℃ for 5 days were inoculated on CAS-substrates with modified Gaus No.1 medium (MGs) and incubated at 28 ℃ for 10 days. The colonies with orange zones were considered as siderophore-producing isolates. The functional groups of the siderophores were determined. The active isolates (width of orange zone on CAS plate >20 mm) were cultured on modified Gaus No.1 broth and incubated at 28 ℃ with shaking at 125 rpm for 10 days. Catechol-type siderophores were estimated by Arnow's method (Arnow 1937) and hydroxamate siderophores were estimated by the Csaky test (Csaky 1948).