In this chapter, we outline a simple procedure using gold nanoshells as a substrate for an immunoassay that is capable of detecting subnanogram levels of analyte within whole blood on the order of minutes. Unique to metallic nanoshells is their optical tunability over a large range of wavelengths. We describe the design of nanoshells that attenuate light strongly in a region of light where blood does not (i.e., the near infrared [IR]), permitting optical detection of nanoshells in whole blood. We also describe a procedure to monitor the analyte-induced aggregation of antibody-conjugated nanoshells in whole blood using near-IR light. The immunoassay is fast and specific, requires no separation/purification steps, is simple to perform (mix and sit), and uses common laboratory equipment for detection (spectrophotometer). Preparation of the antibodynanoshell conjugates is described, along with the design and optimization of the whole-blood nanoshell-based immunoassay system.