A biological approach for producing silver nanoparticles utilizing the proteina- ceous pigment phycocyanin found in cyanobacterial cells has been disclosed. Green silver nanopar- ticles synthesized by Spirulina platensis phycocyanin (SPAgNPs) were tested on three cancer cell lines. SPAgNPs were spherical and crystalline with a size of 30 nm and a net charge of −26.32 mV. Furthermore, they were surrounded by active groups responsible for stability. They scavenged 85% of DPPH radicals with a relative increase of approximately 30% over the extract, while also inhibit- ing the growth of pathogenic microorganisms. Using the MTT assay to measure the progress of cancer cells (A-549 and MCF-7) significantly inhibited them at different concentrations of AgNPs.