The metabolism of 14C-PMM and its irreversible interaction with DNA and proteins were studied in M5076/73A reticular cell sarcoma, a murine solid tumor previously shown to be sensitive to the drug. Metabolism and irreversible binding were determined 0.25, 1, 8 and 104 hours after a single i.p. injection of radiolabelled PMM, tumor and liver macromolecular binding were compared with two differently 14C-labelled PMM, i.e. ring- and methyl-PMM. Ring-PMM derived macromolecular binding appeared to have more relevance in vivo and had a similar time profile in both liver and tumor. Ring-PMM derived DNA binding was then related to metabolic steps between PMM and 2,2,4,6 TMM and 2,2,4,6 TMM itself and 2,4,6 TriMM.