Latices of Euphorbia species utilize labelled acetate and mevalonate for triterpene synthesis under in vitro conditions. Triterpene synthesis was detected at two sites in fractionated latex, viz. in particlecontaining vesicles and in a fraction consisting of amyloplasts, often associated with a tubular membrane system. Attempts to purify these fractions lead to a loss of triterpene synthesis in vitro. Latex particles were successfully purified by means of gel filtration procedures. Radioactive triterpenes were traced in these purified particles after incubation of the latex with (2-14C)acetate or (2-l4C) mevalonate. The newly synthesized triterpenes were firmly attached to the particles of all occurring sizes. Although the fraction containing the amyloplasts was found to be very active in triterpene synthesis, no radioactive latex particles could be isolated from this fraction.