In axenic and mixed cultures, amphotericin B (25-100 mg/L) was tested for inhibition of selected metabolic activities of algae, fungi, bacteria and zooplankton. Effects of the antibiotic were determined based on inhibition of 14-C-labeled bicarbonate uptake, hydrolysis of the methyl ester of 2,4-dichlorophenoxyacetic acid, growth and acid production. Amphotericin B was dissolved in water by either hydrolyzing the antibiotic or mixing it with sodium deoxycholate. Treatments of mixed suspended microbial communities with 200 mg/L of amphotericin B for at least 15 min was effective for differentiating prokaryotic (bacterial) from eukaryotic (algae and fungi, but not zooplankton) activities.