The blue light-excited fluorescent phallotoxin derivative nitrobenzoxadiazole phallacidin (NBD-Ph) was used to stain entire tissue culture monolayers of live L6 mouse cells and other mammalian cell lines without the aid of permeabilization treatment. Although cells tend to exclude the fluorescent toxin, reducing the internal concentration by approximately 1,000 times, some of it enters the cells, probably by pinocytosis, and stains actin structures at low intracellular NBD-Ph concentrations (approximately 5-15 nM), where cell toxicity was negligible or at least not detectable by phase-contrast microscopy. Protracted treatments with NBD-Ph did induce pharmacological responses similar to those of phalloidin. The dissociation constant for NBD-Ph with F-actin in fixed and extracted L6 cells was determined, from staining intensity measurements at various NBD-Ph concentrations, to be 1.5-2.5 x 10(-8) M.