Exosomes are small membrane vesicles secreted by most cell types in vivo and in vitro. Exosomes are found in cell culture media, blood, urine, amniotic fluid, malignant ascite fluids and contain distinct subsets of microRNAs and proteins depending upon the tissue from which they are secreted. Thus exosomes constitute potential biomarkers of human diseases, such as cancer. A major bottleneck in the development of exosome-based diagnostic assays is the challenging purification of these vesicles; this requires time-consuming and instrument-based procedures. Isolation of exosomes can be a tedious, non-specific, and difficult process. Here, we provide a preparative technique for isolation of exosomes based on their ability to aggregate in the presence of lectins. The new method for lectin-based isolation of exosomes from cell culture media was developed as a sample preparation step for exosome-based protein biomarker research.