Nanhumycin (NHM) is a new polyether antibiotic, which suppresses the growth Bacillus subtilis and shows an anticoccidial effect. By means of intracellular recording technique, the effects of NHM on mouse phrenic nerve-diaphragm preparations were observed. The main results are as follows: (1)20 micrograms/ml NHM irreversibly blocked neuromuscular transmission in 1.5-2.0 h. (2) 2-20 micrograms/ml NHM evoked a drastic and spasmodic increase of the frequency of miniature endplate potentials (MEPPs) and a simultaneous increase of the mean quantal content of endplate potentials for 40-60 min, which was followed by complete blockage of EPPs. (3) NHM decreased the resting potential (RP) of muscle cells progressively and irreversibly. (4) NHM's effects on RP or MEPP disappeared in Na(+)-free physiological solution but persisted in the presence of tetrodotoxin (TTX). All these results support the notion that NHM is a cation ionophore, Na+ carrier.