An important attribute of proteome analysis carried out with the aid of two-dimensional gel electrophoresis is that post-translational modifications of proteins can often be revealed. Large-scale proteomic analysis of Streptomyces coelicolor A3(2) has been made possible with the availability of its genome sequence. Here, we bring together observations on the proteins specifically associated with biosynthesis of the isochromanequinone polyketide antibiotic actinorhodin. The predicted products of 14 of the genes annotated as belonging to the act gene cluster were detected. They were generally present only in stationary phase cultures. Plausible explanations are presented for the absence of the other nine. For six of the gene products detected, there was evidence of either specific processing or covalent modification; in the case of the pyran ring closure enzyme ActVI-ORF3, the cleavage of the N-terminal 31 or 34 amino acids was previously shown to be associated with an extracytoplasmic location for the mature gene product. These observations may have implications for the regulation of actinorhodin biosynthesis, and for biochemical studies of artificially expressed Act proteins.