Graphene-based materials (GBMs) have recognized potential for biomedical applications, however different production methods and treatments originate divergent biocompatibility. In this work, two commercially available graphene nanoplatelets (GNP) were studied, differing in platelet size: GNP-C, with 1-2 μm, and GNP-M, with 5 μm. GNP-M was oxidized using KMnO4 in different ratios (1:3 and 1:6), leading to GNP-M-ox-1:3 and GNP-M-ox-1:6. The effect of oxidation and size on biocompatibility was evaluated in vitro. Hemolysis was below 3% for all GBMs from 100 to 500 μg mL-1. GNP-C entered human fibroblasts (HFF-1) inducing reactive oxygen species production after 1 h for 10 μg mL-1, leading to metabolic activity decreases at 24 h, which reverted at 48 h and 72 h. GNP-C was toxic to HFF-1 for 50 μg mL-1. Despite that, GNP-C did not cause damages on cell membrane, opposed to GNP-M and GNP-M-ox-1:3, which were toxic for 20 μg mL-1. GNP-M-ox-1:6 did not decrease metabolic activity or cause membrane damages until 100 μg mL-1 (highest tested) for 72 h. This is explained by complete oxidation causing folding of GNP-M sharp edges, therefore preventing damages. Thus, GNP-M-ox-1:6 has potential for biomedical applications. Equivalent metabolic activity results were obtained for all materials with HPMEC (Human Pulmonary Microvascular Endothelial Cells).