Chlortetracycline is a broad-spectrum antibiotic originally isolated from S. aureofaciens.^1,2 It inhibits growth of both Gram-positive and Gram-negative bacteria at a range of 0.1-100 μg/ml against A. aerogenes, D. pneumoniae, E. coli, K. pneumoniae, P. morganii, and several species of Haemophilus, Neisseria, Salmonella, and Staphylococcus.² Chlortetracycline protects mice from infection by various strains of S. aureus with protective doses (PD50s) of 0.2-7.5 mg/kg, and from infection by E. coli (PD50 = 3 mg/kg) and K. pneumoniae (PD50 = 75 mg/kg).³ It acts by inhibiting protein synthesis, and it binds to a single site on the 30S ribosome subunit.¹ Chlortetracycline is an ionophore and is selective for calcium over sodium, potassium, magnesium, strontium, and barium.⁴ It transports calcium from an aqueous phase into an organic phase environment or into multilamellar vesicles. Chlortetracycline is also a fluorescent dye that can be used to monitor calcium flux.⁵