The contact plate method as used for cod muscle has revealed that the major changes rendering fish unfit for human consumption can be attributed almost entirely to surface pollution of the fish with spoilage bacteria. This is confirmed by three chemical tests, trimethylamine, tyrosine, and surface pH. The relative rates of increase in all three are much greater at the surface than in the interior of cod and haddock fillets. The more rapid surface changes are taken as evidence that tests for spoilage in fish products should be based on samples from the surface of the products and not from composite samples.