Distribution of labeled antibiotics in different components of milk following intramammary and intramuscular administrations.

In crossover trials, four lactating goats were given intramammary infusions and intramuscular injections of radioactivelabeled benzylpenicillin, spiramycin, chloramphenicol, dihydrostreptomycin, and tetracycline. Milk was collected after each treatment and the antibiotic contents in whole milk, skim milk, and whey were determined microbiologically and radiochemically and in cream and casein by radiochemical assay methods. Uptake of antibiotics by cream and casein was highly dependent on drug concentration, increasing with the decrease in antibiotic content in whole milk. Lipophilic chloramphenicol and tetracycline were concentrated in cream to a higher degree than the less lipophilic benzylpenicillin and dihydrostreptomycin. Antibiotic uptake by cream separated from whole milk after intramuscular injection was higher than after intramammary infusion. Antibiotic uptake by casein was independent of the route of administration and was highest for dihydrostreptomycin and tetracycline and lowest for benzylpenicillin.