Suspensions of 35-day-old Mycobacterium tuberculosis H37Rv prepared from stirred liquid cultures and injected intravenously into CD-1 mice accumulated in the lungs at a significantly higher concentration that that seen with logarithmically growing cells. Mice were infected with logarithmic 8-day-old or stationary phase 35-day-old suspensions of H37Rv, and 24 hours later, the bacilli within pooled lung and splenic homogenates were recovered by differential centrifugation. The bacilli were then homogenized in Tween saline and injected intravenously into fresh mice. The partitioning of the 4 inocula into the lungs and spleens of the secondary recipients was compared to that for the original suspensions grown in vitro. There was a significant increase in the number of lung-adapted H37Rv that could again be recovered from the lungs of the secondary recipients compared to that observed for the corresponding splenic preparations. This effect was not due to bacterial clumping or to size differences in the organisms grown in vivo. Homogenation of H37Rv with normal lung increased the relative accumulation of viable bacilli in the lungs compared to the spleens of recipient mice.