Secreted reporters are a useful tool in the monitoring of different biological processes in the conditioned medium of cultured cells as well in the blood and urine of experimental animals. Described here is a protocol for detecting the recently established naturally secreted Gaussia luciferase (Gluc) in cultured cells as well as in blood and urine in vivo. Furthermore, the assay for detecting the secreted alkaline phosphatase (SEAP), the most commonly used secreted reporter in serum, is also presented. The Gluc reporter system has several advantages over the SEAP assay, including a much reduced assay time (1–10 min versus 1.5–2 h), 20,000-fold (in vitro) or 1,000-fold (in vivo) increased sensitivity and a linear range covering over five orders of magnitude of cell number. Additionally, the Gluc signal can be detected in urine and the signal can be localized in animals using in vivo bioluminescence imaging.