Stability of gentamycin in solutions at various pH levels was studied. It was shown that in acid media inactivation proceeded according to the equation of the 1st order reaction. Dependence of the inactivation rate constant on the temperature corresponded to Arrenium equation. A polarimetric procedure for determination of gentamycin providing the assay of the antibiotic levels at various purification stages biginning from the eluates after the ion-exchange columns is described. The results obtained with the above procedure were shown to correlate satisfactorily with those of the microbiological analysis.