Phalloidin is shipped freeze-dried (i.e. in a solid form). Upon arrival it as to be dissolved in methanol (abs.). Depending on the dye the solubility of fluorophore coupled phalloidin is approx.. 0.1 – 0.5 mg/ ml (methanol).
For labeling of mammalian cells a (methanol containing) stock solution of phalloidin needs to be diluted in aqueous buffer to a final concentration of ~20 - 50 µg/ ml. (Depending on the application, cell line etc. phalloidin is used in concentrations between 0.1 and 100 µM in aqueous buffer.)