A quantitative method to assess disinfectant effectiveness was developed. The effects of concentration, pH, presence of serum, temperature, contact time, shelf life, and the influence of medical device material on the effectiveness of liquid disinfecting agents was determined. We also studied the stability of devices made of metal alloys or polymeric plastics after exposure to seven common disinfecting agents. The relative sensitivity to disinfection of thirteen pathogenic bacteria, several viruses (including HIV and Herpes simplex), and standard spores were established. Physical and chemical conditions were found that prevented complete inactivation of pathogens by liquid disinfecting agents. These results may help explain cases of nosocomial infection caused by incompletely decontaminated medical devices, disease outbreaks caused by insufficiently processed food, and epidemics related to inadequate sanitation practices.