The inoculum effect of antibiotic susceptibility testing is often discussed at playgrounds and infectious diseases conferences, but many of us don’t really have a clear definition of what it is or a good understanding of its importance. We’ll be talking about the inoculum effect and susceptibility testing of Staphylococcus aureus with cefazolin. Some of the questions we’ll discuss are: • What exactly is the inoculum effect and how is it measured? • Does the inoculum effect have clinical significance? • How common is the inoculum effect with Staph aureus and cefazolin?