One hundred sixteen clinical isolates of Enterobacter sp., Escherichia coli, Klebsiella sp., Proteus mirabilis, Staphylococcus aureus and S. epidermidis were tested against cefoperazone, cefoxitin, cefamandole and piperacillin by disc-agar diffusion. Zones-of-inhibition of discs dropped near the center of the plate were compared with those dropped near the periphery. In 28% of the comparisons the inner and outer zones differed by 2 mm or more. Of those, the inner zone was larger 2.5 times more often than the outer zone. Results suggest that the chances of obtaining a susceptible result are greater when discs are placed near the center as opposed to the periphery of the plate.