3H-thymidine is injected to Wistar rats 60 min before their sacrifice, that is performed 24 h after intratracheal administration of isotonic solution of natrium chloride (control), and in the test animals--24 or 72 h after similar injection of macrophage decay products (MDP). The MDP are prepared by freezing-thawing of syngenic peritoneal macrophages repeated 5 times in the dose corresponding to 10(8) of the destroyed cells per 100 g of body mass. During the first period after MDP injection the labelling index of macrophages, histiocytes and cells of the alveolar epithelium increases up to 4.07 +/- 0.21%, during the second--up to 3.34 +/- 0.17% (in the control--1.90 +/- 0.18%). The results obtained are discussed according to modern ideas on the role of the interstitial cell pool as an immediate source for mobilization of alveolar macrophages, and together with the data previously accumulated, on the role of MDP in the autoregulatory control of various links of the macrophagal pulmonary reaction.