A simple, fairly rapid, sensitive and accurate method is described for the colorimetric determination of neomycin sulphate (NMS), based on the measurement of the absorbance of the extracted organic soluble ion-association complex formed between neomycin dictation and a bulky counter anion. Different chromotropic acid azo dyes were examined as counter ions. The effect of pH, the counter ion concentration, sequence of addition and solvents for extraction were also illustrated. The most suitable system is based on reagent VIII (pH 7.5) with chloroform as the extraction solvent. The use of other counter ions, in conjunction with their respective solvents, was found to be less sensitive. The neomycin-reagent VIII system exhibits negligible or no interference when used for the determination of up to 58 microgml(-1) of NMS in the presence of several drug excipiences. The method has been used for the determination of up to 58 microgml(-1) with a good recovery (99.8+/-1.5%), and the precision is supported by the low relative standard deviation </=1.35%. The sensitivity is discussed and the results are compared with the official method. The proposed method was applied successfully to the determination of NMS in pure and dosage forms, with a good precision and accuracy compared to the official one.