A rapid method for determining the sensitivity ofStaphylococcusaureus to β‐lactarns is described. The method involves the culture of microcolonies of the bacterium on the surface of antibiotic‐containing agar medium on microscope slides. The areas of the microcolonies formed are measured by means of a microscope and image analyzer. The sensitivity of the bacterium can be estimated in 4 h by this technique and the results are comparable to those obtained by overnight incuba