Compositions and Methods for Stabilizing Circulating Tumor Cells Methods and compositions for stabilizing a biological sample for analysis, comprising the steps of obtaining in a sample collection device a biological sample from a subject, especially blood, the biological sample including at least one circulating tumor cell from the subject. The methods may include a step of contacting the biological sample with a protective agent composition that includes a preservative agent, an optional anticoagulant, and a quenching agent to form a mixture that includes the protective agent composition and the sample.