A colorimetric procedure was developed for the quantification of gentamicin. The method was based on the ninhydrin reaction with primary and secondary amines present in the gentamicin. This reaction produces a purple colour. The effects of several factors including pH, ninhydrin concentration and reaction time were investigated to optimize the assay method. Using the assay protocol, the absorption of the gentamicin-ninhydrin mixtures at 400 nm had a linear relationship with the gentamicin concentration ranging from 30 to 120 microg/ml. The colorimetric gentamicin assay reported herein is of great practical value because it is reproducible, sensitive, simple and extremely inexpensive.