Leukotriene formation, which is mediated by 5-lipoxygenase (5-LOX), and the oxidation of low-density lipoproteins (LDLs) have been shown to contribute to the development of atherosclerosis. Earlier studies have shown that both mechanisms are inhibited by polyphenols, especially anthocyanins. The rutinoside-conjugated anthocyanidins present in black currants (Ribes nigrum L.) have higher bioavailability than monosaccharide-conjugated anthocyanidins and may therefore be beneficial in reducing the risk of cardiovascular diseases. We investigated the in vitro inhibitory potentials of cyanidin-3-O-rutinoside and delphinidin-3-O-rutinoside against 5-LOX activity and LDL oxidation. Both anthocyanins were able to significantly inhibit 5-LOX activity and LDL oxidation. The investigated aglycones, cyanidin and delphinidin, showed a lower 5-LOX inhibitory potential, but were more potent at inhibiting LDL oxidation than their respective rutinosides. Furthermore, the inhibitory potentials of certain colonic degradation products of anthocyanins (protocatechuic acid, phloroglucinol aldehyde, vanillic acid, and gallic acid) were assessed. Gallic acid stimulated 5-LOX activity and was the only degradation product that was able to inhibit LDL oxidation. The other degradation products inhibited 5-LOX activity at high concentrations. Anthocyanins, anthocyanidins, and several of their degradation products demonstrated in vitro inhibition of mechanisms that contribute to atherosclerosis development.