Using transplanted mussels as an in-situ bioassay to assess marine environmental quality has provided important information on bioavailability of contaminants and associated bioeffects that would not have been available with traditional chemical monitoring, biomonitoring, or laboratory bioassays. It is one of the most promising field bioassay systems because of the relative ease in making synoptic measurements of bioaccumulation and growth to estimate chemical exposure and bioeffects, respectively. In-situ field studies that utilize transplanted animals combine the advantages of environmental realism associated with field monitoring and experimental control associated with laboratory testing. Because they are sedentary and concentrate contaminants, resident mussels have been used extensively to estimate exposure by measuring contaminants in their tissues. The use of resident mussels as a response indicator has been very limited because of the difficulties associated with measuring biological processes and defining the exposure period in natural populations. Transplanting caged mussels facilitates measurements of bioeffects and clearly defines the exposure period. Data from a series of mussel (Mytilus galloprovincialis) transplants in San Diego Bay, California, demonstrate how this methodology can be used to assess the extent of contamination and associated biological effects. Although more than 18 sites were studied in nine separate transplants between 1987 and 1990, emphasis-will be placed on two sites in the Shelter Island Yacht Basin separated by only 3 meters vertical distance. The mussel field bioassay was used to identify the following: (1) site-specific differences, (2) temporal and spatial variability, (3) short-term and long-term trends, (4) potential sources of contamination, and (5) dose-response relationships.