Protein-protein interactions and protein complex/aggregate formation play an essential role in almost all biological functions and activities. Through a nanoparticle aggregation immunoassay, we discovered that some proteins are substantially more complexed/aggregated in cancer tissues than normal tissues. This study examined four biomarkers proteins, CA125, CEA (carcinoembryonic antigen), CA19-9 and PAP (prostatic acid phosphatase) in ovarian, colon and prostate tissue lysates. The most exciting results were observed from the PAP assay of prostate tissues prostate cancer can be clearly distinguished from normal prostate and prostate with benign conditions such as BPH (benign prostate hyperplasia) based on the complex/aggregation level of PAP in prostate tissue lysates. The complex/aggregate level of a protein can be potential biomarkers for cancer detection and diagnosis.