Storage treatments (12°C, 95% RH) were applied to freshly harvested mushrooms for 0, 24 and 72 hr. Canned product yields were determined on replicated samples processed by a standardized procedure. Canned product yields increased 19% after 72 hr storage and total free amino acid content of the tissues increased proportionately. Electrophoretic analyses employing sodium dodecyl sulfate gels demonstrated that degradation of larger molecular weight proteins occurred during storage with a corresponding increase in the lower molecular weight fractions. Water-binding capacity and water-holding capacity of partially purified proteins prepared from treated lots increased as storage time progressed. Data substantiated the hypothesis that qualitative changes in mushroom proteins are a factor in the increased product yields obtained by postharvest storage of mushrooms.