The antibacterial activity of LY146032 was greatly potentiated in the presence of calcium ions. In the presence of a physiological concentration of calcium (ca 100 mg/l: 2.5 mM) the new compound was more active than vancomycin or teicoplanin against a selection of clinical isolates of staphylococci and streptococci. The requirement for calcium could not be satisfied by magnesium. The activity of LY146032 varied when tested in different culture media, including batches of Mueller-Hinton agar from different manufacturers. The highest minimum inhibitory concentrations of LY146032 were observed in tests on Iso-Sensitest agar, which contains little calcium. Supplementation of Iso-Sensitest agar with increasing concentrations of calcium caused a proportionate fall in the MIC of LY146032. Saponin-lysed horse blood; incubation in a CO2-rich atmosphere; and an increase in the bacterial inoculum from 10(3) to 10(6) cfu/spot had little effect on the activity of LY146032 in the presence or absence of calcium.