In this study, 24 marine macroalgae collected from the coastline of Izmir Gulf were examined for their antioxidant activities and their effects on cell proliferation. Crude extracts were obtained from samples with cold methanol extraction. Antioxidant activity was evaluated as 2,2-diphenyl-1-picryl-hydrazyl (DPPH) radical scavenging activity and total phenolic content (TPC); growth inhibitory effects of the extracts were determined by using WST-8. Amongst the species, Polysiphonia denuata (Rhodophyta) and Cystoseira species (Phaeophyceae) have been noticed by their high DPPH radical scavenging activities and TPCs. As expected, there was a strong correlation between these tests. Dictyota dichotoma (Phaeophyceae) showed the highest anti-cancer activity on MCF-7 cells with an IC50 of 17.2 ng mL(-1). Flow cytometry analyses demonstrated that D. dichotoma methanolic extract strongly induced apoptosis. This extract exhibited moderate viability inhibition on MCF10A cells (IC50: 49.3 ng mL(-1)), suggesting a potential use of the extracts or its compounds for cancer therapy. There was no correlation between anti-cancer potential and antioxidant content of the extracts.