A series of dimeric procyanidins (1-9) and some related polyphenols (10-15) were chosen as model compounds in a comparative investigation for various biological activities in order to obtain structure-activity relationships. Antiviral [herpes simplex virus (HSV) and human immunodeficiency virus (HIV)], antibacterial, superoxide radical-scavenging, and complement-modulating properties were assessed. In general, more pronounced activities were seen with epicatechin-containing dimers for anti-HSV, anti-HIV, and radical-scavenging effects, while the presence of ortho-trihydroxyl groups in the B-ring was important in compounds exhibiting anti-HSV and radical-scavenging effects and complement classical pathway inhibition. Double interflavan linkages gave rise to interesting antiviral effects (HSV and HIV) and complement inhibition. The influence of the degree of polymerization or the type of interflavan linkage (4-->6 or 4-->8) differed in the different biological systems evaluated. Only minor or moderate antibacterial effects were observed for the compounds under investigation.