Studied was the use of monensin-sodium Pharmachim (MSP) in the form of a 10 per cent premix. The tolerance of the preparation was followed up along with the subchronic toxicity and the residual amounts with pigs. It was established that amounts of 10, 20, and 30 mg/kg body mass at single application produced a lethal effect. However, medicated feeds containing MSP at the rate of 60, 120, and 240 ppm offered in the course of 30 days to pigs did not produce any negative effects on the behaviour, appetite, gain, white blood picture, and the biochemical indices of blood under study. On the other hand, a concentration of 240 ppm suppressed haemopoiesis, while those of 60 and 120 ppm enhanced the gain of pigs. No gross morphologic lesions were found with the use of the three concentrations. Histologically, changes were seen only in pigs that were given MSP in concentration of 240 ppm, consisting in but slightly manifested degeneration of the epithelium of kidney tubuli contorti. The withdrawal times for pigs treated for 30 days with MSP in conc. of 60 and 120 ppm was 18 hours, and for those given 240 ppm of the preparation--42 hours.