A new phenylalanine ammonia-lyase (PAL) was purified from leaves of Ocimum basilicum L. (chemotype, methyl cinnamate). Separation techniques applied included anion exchange chromatography and preparative electroelution from a non-denaturing polyacrylamide gel. A 180-fold purification was obtained. The native enzyme was a homotetramer of M r 152 000-153 000; the intact subunit M r was ca 38 000. The enzyme catalysed the conversion of l-phenylalanine-d 8 into trans-cinnamic acid-d 7 , as determined by GC-mass spectral analysis of silylated reaction products. The purified native enzyme had K m and V m a x values of 329 μM and 11.43 μ mol min - 1 mg - 1 protein, respectively, for l-phenylalanine and was competitively inhibited by 2-amino-indan-2-phosphonic acid, trans-cinnamic acid and trans-methyl cinnamate with K i values of 19 nM and 57 and 130 μM, respectively. Comparing the K i values between trans-cinnamic acid and trans-methyl cinnamate forl -phenylalanine indicated that the regulation of PAL is not only related to the mechanism of feedback inhibition in the biosynthesis of trans-methyl cinnamate.