Microorganisms possess stringent cell membranes which limit the cellular uptake of antimicrobials. One strategy to overcome these barriers is to attach drugs or research reagents to carrier peptides that enter cells by passive permeation or active uptake. Here the short endocytosis signal peptide NPFSD was found to efficiently deliver both FITC and GFP,into Saccharomyces cerevisiae and Candida albicans with uptake into the majority of cells in a population. The NPFSD signal is itself non-toxic, but when fused to the ricin A chain toxin (RTA) the peptide enhanced both cell uptake and toxicity against C albicans, which like other yeasts is resistant to naked RTA. Cell entry required at least 1 h incubation, temperatures above 4degreesC, and an energy source, and uptake was outcompeted with free peptide. Therefore, the NPFSD peptide can carry a range of compounds into yeasts and this delivery route holds promise to enhance the activity of antifungals. (C) 2003 Elsevier Science (USA). All rights reserved.