A new alkaloid, perlolyrine, 1-(5-hydroxymethyl-2-furyl)-9H-pyrido[3,4-b] indole (6b), has been isolated from Lolium perenne L. (Graminae). The structure of the base was revealed by an X-ray analysis of its hydrobromide dihydrate C16H12N2O2,HBr,2H20 (space group P1, a=7.81, b=10.55, c=10.55 Å, α=79.2 α= 72.9 β, γ= 78.8°), by use of the heavy-atom method, assisted by spectroscopic data. The final value of R over 2360 reflections is 0.129; an attempt to refine the structure in space group P1 failed. The alkaloid has been synthesised from tryptophan and 5-acetoxymethyl-2-formylfuran. The 3,4-dihydro- and 1,2,3,4-tetrahydro-derivatives have also been synthesised, starting from tryptamine. The ways in which perlolyrine and its reduced derivatives break up on electron impact are discussed. Perlolyrine is shown to be present in the dried grass, and is not an artefact of the working-up process.