Co1–xMnxFe2O4 ferrite nanoparticles have been synthesized by employing a microwave‐stimulated technique under non‐hydrolytic conditions. Structural, chemical, and morphological characterizations were carried out by XRD, SEM‐EDS, and TEM techniques. The stability of the nanoparticles and their tendency to form agglomerates were investigated by dynamic light scattering, exploiting the effect of different biological environments and protein stabilization. It was shown that bovine serum albumin adsorption was dependent upon the concentration and chemical composition of the nanaoparticle, showing higher affinities with increasing Co2+ content. The cytotoxicities of the nonsurface‐blocked Co1–xMnxFe2O4 nanoparticles were assessed for three cell lines: phagocytic murine macrophage (J774.E), cancer fibroblast human osteosarcoma (HTB), and mesenchymal stem cells derived from human adipose tissue (ASCs). The results indicate that the cytotoxic response strongly depends on the particle concentration as well as the type of cell line.