1. The stereoisomers of delta-(alpha-aminoadipyl)-L-cysteinylvaline (LLD, LLL and DLD) were synthesized from valine labelled with 3H in its methyl groups or in the alpha position. L-Cysteinyl-D-[4,4′-3H]valine was also synthesized. 2. 3H was incorporated into a compound that behaved like penicillin N when the LLD tripeptide containing either a methyl- or an alpha-labelled valine residue was incubated with a cell-free system prepared by lysis of protoplasts of Cephalosporium acremonium. 3. Incorporation was not observed under these conditions from the labelled all-L- or DLD-tripeptide, from L-cysteinyl-D-[4,4′-3H]valine, or of Penicillium chrysogenum appeared to be the LLD isomer, like that from C. acremonium. 5. These findings are discussed in relation to penicillin biosynthesis.

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