The blue copper proteins are among the most beautiful macromolecules that we know, and the intensity of the colour of azurin per copper atom is eighty times as that of the cuprammonium ion. They occur in the periplasm of Gram positive bacteria and in the photosystems of blue green algae (cyanobacteria) and in algal and higher plant chloroplasts. The function of these proteins in electron transport is very similar to those of the soluble cytochromes C-6 ("algal cytochrome f") and c(6) ("Pseudomonas cytochrome c-551"), so much so that one thinks of them as 'honorary cytochromes'. Several different sequence classes have been recognized, including azurin, plastocyanin, amicyanin, pseudoazurin and rusticyanin. There is considerable three-dimensional similarity in the copper binding sites of the proteins, and have as ligands two histidine, one cysteine and one methionine side chains. The proteins have been co-opted to function in a wide range of metabolisms, and azurin is often (but not always) associated with denitrification. The gene has been adapted to produce an outer membrane protein in gonococci, while azurin, amicyanin and pseudoazurin are involved in methylotrophy in different organisms - though in other closely;related bacteria, cytochromes c(8) play equally versatile roles. While the overall structures of each of the proteins are very similar wherever they are found, when duplicate genes occur in the same organism, the difference between the isoforms are large, even if no functional difference between the forms has been detected in vitro. The time is ripe and the techniques now available for the function and relationships of these proteins to be elucidated genetically, which has not been possible before because of the multiplicity of alternative pathways in bacterial electron transport systems.