On the occasion of the first international symposium on pyrroloquinoline quinone (PQQ) and quinoproteins (Delft, September 1988), a review of this novel field in enzymology is presented. Quinoproteins (PQQ-containing enzymes) are widespread, from bacteria to mammalian organisms (including man), and occur in several classes of enzymes. Indications already exist that PQQ is a versatile cofactor, involved not only in oxidation but also in hydroxylation, transamination, decarboxylation and hydration reactions. The current list of quinoproteins shows that it was overlooked in several well-studied enzymes where the presence of a common cofactor had already been established. Up until now, all eukaryotic quinoproteins have covalently bound PQQ (or perhaps pro-PQQ), while free PQQ occurs exclusively in a number of (bacterial) dehydrogenases and in the culture fluid of certain Gram-negative bacteria. Biosynthesis of free PQQ in methylotrophic bacteria starts with tyrosine and glutamic acid as precursors while intermediates in the route have not been detected and the presence of free PQQ is not required for synthesis of the covalently bound form of the cofactor in glutamic acid decarboxylase from Escherichia coli. Therefore, the assembly of covalently bound cofactor might occur in situ, i.e. in the quinoproteins themselves. If the latter also applies to mammalian quinoproteins, this implies that PQQ is not a vitamin. On the other hand, positive effects have been reported upon administration of PQQ to test animals. Methods suited to detach and to detect PQQ with a derivatized o-quinone moiety may answer questions on the uptake and processing of the compound.