Pulsed field gradient nuclear magnetic resonance (PFG-NMR) was used to measure tracer diffusion coefficients of vitamin B-12 and globular proteins (myoglobin, beta-lactoglobulin, ovalbumin, and bovine serum albumin) in semidilute alginate solutions under physiological conditions (pH 7.4, ionic strength 0.22 M). The long-time diffusion behavior can be predicted by an obstruction-scaling model, which assumes that the semidilute polymer solutions can be treated as a static network during the diffusion time scale, and the contributions of hydrodynamic interactions and electrostatic interactions to diffusion can be neglected. Arguments are given in support of these assumptions, which are expected to be valid only when the size of the probe is much larger than the polymer chain. The model was also applied to literature data of protein diffusion in flexible and inflexible polymer solutions, and it was found to be predictive. Moreover, it is demonstrated that, at equal polymer solution mesh sizes, probe diffusion is not influenced by the polymer chain flexibility.