Blood serum levels and half-life times of Rifampicin (RAMP) have been investigated after the first administration, at various time intervals during long-term treatment and following a test dose given 30 days after the end of long-term treatment in tuberculous patients. After a marked drop of the values at the 6th and 12th hour following administration, which takes place within the first 30 days of treatment together with a sharp decrease in the half-life times, no further appreciable changes have been observed up to the 10th month of treatment. After a test dose given 30 days after the end of treatment, a curve comparable to that observed following the first administration is obtained. The changes are likely to depend upon an increased metabolization of the drug to its desacetyl-derivative which is more rapidly excreted and very poorly absorbed. This behaviour does not affect to any appreciable extent the therapeutic effectiveness of RAMP.