This work is an extension of our previous work (Hall et al., 1993) on the synthesis and cytotoxic activity of boronated peptides. The aim of this work was to carry out structural modifications of the amine terminal in compounds1 and2, to increase water solubility, and its effect on the cytotoxicity to tumor cell lines. Surprisingly, only compounds4,7 and8 were more water soluble than the parent compounds. With the exception of compound4, the new derivatives were generally less effective than the parent compounds (1 and2). There was no apparent correlation between structure and activity. Cytotoxic effect was more pronounced in single cell suspended cells. The growth of solid tumor cell lines was not significantly reduced. The most active derivative, (methanamine)dihydro[[[1-(phenylmethyl)-2-methylamino-2-oxoethyl]amino]carboxy]boron (4), inhibited DNA, RNA, and protein synthesis in Tmolt3 cells. Enzymatic activities, e.g., DNA polymeraseα, m-RNA polymerase, PRPP amidotransferase, carbamyl phosphate synthetase, TMP-kinase, TDP-kinase, dihydrofolate reductase, and ribonucleoside reductase were reduced after 60 min incubation with4. d(TTP) and d(CTP) pool levels were also reduced by 60 min incubation with4.