This study investigated the stability and transfection efficiency of plasmid DNA (pDNA) and sea urchin sperm histone H1 (Sp H1) complexes embedded in albumin microsphere formulations. Sp H1 increased the stability and transfection efficiency of pDNA, while providing a favourable sustained pDNA release profile. Encapsulating Sp H1-complexed pDNA into albumin microspheres further protected the pDNA from physical stress and heparin treatment. When compared with free pDNA encapsulated in albumin microspheres, the Sp H1-pDNA microsphere formulations exhibited decreased hydrophilicity, slower pDNA release profiles, protection against heparin-induced degradation of embedded pDNA and increased stability against physical stress. These results indicate that complex formation of pDNA with Sp H1 facilitates intracellular DNA transfer and that albumin microspheres-Sp H1-pDNA gene delivery formulations are suitable for controlled-release delivery of pDNA while offering protection of the pDNA from degradation and maintaining pDNA biological activity.