Experiments were conducted to ascertain whether the antitumour activity of hexamethylmelamine analogues correlated with their in vitro cytotoxicity and metabolism. Two analogues, namely pentamethylmelamine (PMM) and 2,2,4,4-tetramethylmelamine (TMM), and hexamethylmelamine (HMM) itself were shown to be active towards the murine ADJ/PC6A (PC6) plasmacytoma; another three, 2-chloro-4,6-bis(dimethylamino)-1,3,5-triazine (CBDT), 2,4-bis-(dimethylamino)-6-hydrazino-1, 3,5-triazine (HBDT) and 2,4,6-trimethylmelamine (TriMM) were inactive against the same tumour. The cytotoxicity of these compounds was examined against a PC6 tumour cell line in vitro. In the absence of liver microsomal activation only CBDT proved to be significantly cytotoxic at a concentration of 5 mM. In the presence of murine liver microsomes the three active antitumour agents were all cytotoxic at this concentration whereas HBDT and TriMM remained non-toxic. The degree of cytotoxicity correlated with the extent of metabolism for these analogues. The products of biotransformation of these compounds were stable precursors of formaldehyde (presumably N-hydroxymethyl intermediates) (FP) rather than formaldehyde itself. After injection of these 6 compounds to Balb/c mice the levels of FP generated in the plasma were markedly greater for the three active antitumour agents than for the inactive analogs. No free formaldehyde was detected in the plasma after administration of any of the compounds. These results suggest that for these compounds in vitro cytotoxicity correlates with in vitro biotransformation and their antitumour activity correlates with plasma levels of FP generated by metabolism in vivo.