To test the hypothesis that the in-vivo antibiotic action of tetracycline might be affected by ferric iron and the enhancement of infection by ferric iron by tetracycline, the actions of intraperitoneal antibiotic and local ferric ammonium citrate, given separately and together, were measured in the dorsal skin of guinea-pigs bearing lesions due to staphylococci, streptococci, a sp., an sp., and

Tetracycline, given in two intraperitoneal doses of 25 mg/kg at 0 and 2 h after intracutaneous challenge, maintained plasma concentrations of 4–6 μg/ml for more than the first 4 h of infection, after which the local lesions had become largely insusceptible to the antibiotic. The intracutaneous injection of Fe 10 μg in a volume of 0.1 ml containing the bacteria was sufficient to enhance infection by those strains susceptible to this effect.

The in-vivo efficacy of tetracycline was not always related to low MIC; a low MIC was sometimes associated with little action and a high MIC with moderate action.

Sixteen organisms were tested. The iron diminished the tetracycline effect only feebly with one staphylococcal strain and the strain of In only one case, with a strain of sp., was the tetracycline action grossly diminished. On the other hand, tetracycline diminished the enhancement effect of iron moderately with three strains of staphylococci and one strain each of and , and strongly with two strains of staphylococci, a group-C streptococcus and one strain each of and

It is evident that the diminution of tetracycline action by moderate excess of readily available Fe, whether endogenous or administered, is an unlikely event (three instances among the 16 tested) whereas the diminution of the infection-enhancing effect of iron by tetracycline is much more likely (12 instances among the 16). Insofar as a decrease in iron available for enhancement of infection is valid evidence of a


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