The bleomycins, phleomycins and tallysomycins are structurally similar glycopeptide antibiotics. Within each class, individual members differ only in the structure of a basic group. The antibiotic effect of phleomycin (Bristol batch A9331-648) against Escherichia coil is amplified substantially by a number of simple heterocyclic and aromatic compounds. In this paper a sample of 26 such compounds were tested for this property with 25 different phleomycins, bleomycins and tallysomycins. The nature of the variable basic group of the phleomycins, bleomycins and tallysomycins determined the response obtained with all amplifiers, although variation of response was much less marked with caffeine which potentiated the cytotoxic effects of all the phleomycins, bleomycins and tallysomycins tested. Phleomycins and bleomycins having two or three guanidino groups in the variable basic group, or phleomycins having a secondary amino group within a methylene chain and a terminal 2-phenylethyl substituent, were amplified by most compounds, whereas the cytotoxicity of others was enhanced little or not at all. Similar phleomycins, having a secondary amino and a terminal guanidino group and no 2-phenylethyl substituent showed little enhancement, and in these cases the inclusion of a 2-phenylethyl substituent had a major influence in determining amplifiability. Bleomycins and phleomycins having identical basic groups were amplified to similar extents by the sample of 26 amplifying agents used.