Pure and composite hydrogel matrices of collagen type I and fibrin were produced by simultaneous polymerization of each biopolymer in the presence of vascular smooth muscle cells. The ratio of collagen to fibrin in composite matrices was varied from 1:1 to 1:4, with corresponding absolute protein concentrations of 1.0-5.0 mg/mL. Constructs cultured for 7 days were subjected to uniaxial tensile testing, analysis of cell content, as well as scanning electron and confocal microscopic imaging. Gel compaction over time in culture decreased with increasing protein content but was augmented by the presence of fibrin. Material properties (modulus, ultimate tensile stress, and toughness) were highly correlated with gel compaction, protein density, and cell concentration. Maximum force at failure was dependent on absolute protein concentration. This study examined the interrelationships between protein type, ratio, and density in composite biopolymer matrices and contributes to the understanding of structure-function relationships in such materials.