The toxicity of a number of solubilized or colloidal amphotericin B formulations to human erythrocytes has been studied in-vitro. All the solubilized formulations studied, using poloxamer F127 or L92, or sodium deoxycholate as solubilizing agents, showed similar toxicity, erythrocyte lysis being greater than 90% for amphotericin B concentrations between 4 to 8 micrograms mL-1. Emulsion formulations stabilized by poloxamers showed reduced toxicity, while those stabilized by egg lecithin showed less than 5% erythrocyte lysis up to an amphotericin B concentration of 200 micrograms mL-1. The mechanisms of the differential toxicity is considered to be due to the differences in the equilibrium concentration of free amphotericin B in the aqueous phase.