Nanoparticles of Cu–Cd–Ni–FeO₄ weres ynthesized by the auto-combustion process using a modified citric acid method. Antibacterial activity, structural, dielectric and magnetic properties were investigated. An assay showed the broad spectrum antibacterial activity of Cu–Cd–Ni–FeO₄ against Gram positive and Gram negative pathogenic bacterial strains. The existence of single phase cubic spinel structure of ferrites was confirmed by X-ray diffraction measurement. Fourier transform infrared spectroscopy shows the two main absorption bands at lower frequency region. Surface morphology and compositional features were studied by scanning electron microscopy and energy dispersive X-ray spectroscopy analysis, respectively. Results indicated that the nanosize particles greatly influenced the antibacterial activity, as well as structural, dielectric and magnetic properties of the samples. Magnetic measurements of the samples were carried out by means of vibrating sample magnetometry and Mössbauer spectroscopy. Magnetic properties are strongly affected by Ni²⁺ substitution.