Drug access to tumors is limited by diffusion through the tumor interstitium. We used a microfiberoptic epifluorescence photobleaching method to determine the role of extracellular matrix (ECM) components in macromolecule diffusion deep in tumor tissue. In subcutaneous B16 tumors in living mice, translational diffusion of 10 kDa FITC-dextran was slowed 2- to 3-fold (compared with its diffusion in water) within a depth of 0.2 mm from the tumor surface, but >10-fold beyond a depth of 1 mm. Diffusion of larger macromolecules, FITC-albumin and 500 kDa FITC-dextran, was slowed by up to 40-fold at 0.5 mm and 300-fold at 2 mm. Intratumoral collagenase (to digest collagen) or cathepsin C (to digest decorin) each increased diffusion of 10 kDa FITC-dextran by approximately 2-fold. However, these treatments dramatically increased diffusion (>10-fold) of larger macromolecules, such as 500 kDa dextran, in deep tumor (2 mm depth). Intratumoral hyaluronidase, in contrast, slowed diffusion throughout the tumor. In vitro measurements in defined gel-like mixtures of collagen, hyaluronan, and decorin closely recapitulated results in tumors in vivo. Mathematical modeling quantified the roles of extracellular space volume fraction and dimensions, and indicated a substantial effect of cell density on diffusion in deep tumor. Our data define the determinants of diffusion in deep tumor and suggest collagen and decorin digestion to greatly facilitate macromolecule delivery.