Abstract: An optical biosensor carries one or more affinity legends or binding members that bind specifically to a marker being monitored. Light directed along optic fibers illuminates a surface plasmon resonance (SPRN) probe surface on which is immobilized the binding member. A spectrophotometer receives light reflected back along the fiber optic path and provides wavelength information indicative of the absence or presence of surface plasmon resonance indicative of the bound marker in known SPR manner. The probe is used in vitro or in vivo. When used in vivo the fiberoptic light path comprises a catheter that directs the probe to an implant site. For in vivo implantation a housing houses the probe at the implant site and is adapted to filter out larger particles that would adversely affect with the spectral analysis. In one embodiment the probe has two regions on its surface. The first region has no immobilized binding member. The second region does have the binding member immobilized on it.