Twelve patients receiving intermittent treatment with rifampicin for pulmonary tuberculosis, who had developed rifampicin-dependent antibodies were studied. Ten of the twelve patients had also developed adverse reactions to the drug at the time the study was done. After a dose of the drug, blood samples were taken from each patient at regular intervals for the assessment of rifampicin-dependent antibodies by a semi-quantitative method, and for the measurement of serum rifampicin concentrations. Clinical details of any adverse reactions which occurred were also recorded at these set times. Serum rifampicin concentrations were at their highest from 1 to 12 hr after a dose, and during this same period the rifampicin-dependent antibody score was reduced. Adverse reactions, when they occurred, coincided with the period of high serum rifampicin concentration and low antibody score. Whether or not these findings are of immunological significance is discussed.