Triacetyloleandomycin and josamycin, when administered to inbred Balb/c mice orally twice daily at a dose of 12.5 or 25 mg per kg over 3 days, were found to increase barbiturate sleeping time significantly. In contrast, erythromycin ethylsuccinate, erythromycin propionate, midecamycin and spiramycin were devoid of any such activity under the same conditions.