We report here, with respect to collagen production and the mechanical properties of a fibrin‐based media equivalent (ME), on our efforts to optimize the culture conditions of neonatal SMCs entrapped in tubular fibrin gels. We examined several factors, including the concentration of fibrinolysis inhibitor, the cell source and initial number, the addition of TGF‐β and insulin to the culture medium, and the time in culture. We found that varying the concentration of ϵ‐aminocaproic acid (ACA), an inhibitor of fibrinolysis, did not affect the collagen production, but that lower concentrations resulted in a compromised physical integrity of the ME. While use of neonatal SMCs yielded superior results over adult SMCs, a higher initial cell number did not improve results. The addition of 1 ng/mL of TGF‐β to the medium increased the collagen content fourfold and the ultimate tensile strength (UTS) and modulus approximately tenfold after 3 weeks, while the addition of both TGF‐β and insulin improved collagen content sixfold and UTS and modulus almost 20‐fold. Additional TGF‐β (5 ng/mL) did not improve any of the properties measured, but additional time in culture did. Samples incubated for 6 weeks with TGF‐β and insulin contained about seven times the amount of collagen and had a three‐times higher UTS and modulus than did samples incubated for only 3 weeks. When compared to collagen MEs, the fibrin MEs compacted to a greater extent and were both stronger and stiffer when cultured under the same conditions, having after 6 weeks a tensile modulus and ultimate tensile strength similar to those of rat abdominal aorta. © 2003 Wiley Periodicals, Inc. J Biomed Mater Res 66A: 550–561, 2003