Levels of erythromycin in the blood and organs of rabbits, rats and mice were determined after the i. m. implantation of the drug Erythromycin inj. ad usum vet. at doses of 10 mg kg-1 live weight (rabbits), and/or 25 mg kg-1 (rats) and 1 mg pro toto--approximately 40 mg kg-1 (mice). A diffusion plate method and the germ Sarcina lutea CCM 552 were used to find out the antibiotic in samples. Sensitivity of the method used was 0.025 microgram ml-1 (g). In rabbits the level of the antibiotic was maintained in the blood and examined organs and tissues (lungs, liver, kidney, muscles and the wall of the small intestine) by the above dose for more than 12 hours--except liver. The values obtained for the organs and tissues were mostly higher than those obtained for the blood. The highest levels were found in the kidneys and lungs. Neither did the dose of 20 mg kg-1 l. w. maintain the significant level of erythromycin in the blood serum in the course of 24 hours. The results obtained in rats and mice almost coincided with the distribution of erythromycin in the rabbit organisms, and the affinity of this antibiotic with the pulmonary tissue was proved.