Uroporphyrinogen III methylase was purified from a recombinant hemB-strain of E. coli harbouring a plasmid containing the cysG gene. N-terminal analysis of this purified protein gave an amino acid sequence corresponding to that predicted from the genetic code. From the u.v./visible spectrum of the reaction catalysed by this SAM dependent methylase it was possible to observe the sequential appearance of the chromophores of a dipyrrocorphin and subsequently of a pyrrocorphin. Confirmation of this transformation was obtained from 13C-NMR studies when it was demonstrated, for the first time directly, that uroporphyrinogen is initially converted into dihydrosirohydrochlorin (precorrin-2) and then, by further methylation, into a novel trimethylpyrrocorphin.