Long range surface plasmon resonance (LRSPR) was used to improve over standard SPR for the detection and monitoring of toxicity with living cells. With a greater evanescent field penetration depth into the sensing medium and increased sensitivity to optical scattering, LRSPR shows a higher sensitivity for measurements involving molecular content redistribution associated with cellular morphology changes. LRSPR and SPR sensitivity to real and imaginary refractive index component variations was evaluated separately using calibrated oil solutions and dilutions of polystyrene beads, respectively. An increase in reflectance at the minimum along with a broadening of the curve was observed for LRSPR as a result of evanescent field scattering. Angular scans for SPR were marginally affected, due to an evanescent field too shallow for the beads to produce significant scattering. Methylene blue dilutions in water were used to evaluate sensitivity to complex refractive index variations. Finally, LRSPR and SPR were used to measure the response of a HEK-293 cell monolayer under stimulation at different concentrations by a toxin, lipopolysaccharides. The experimental results presented confirm that living cells must be modeled as a dielectric medium with a complex refractive index in LRSPR measurements and that LRSPR exhibits a 50% greater sensitivity compared to standard SPR for toxicity measurements based on living cells.