The growth of coagulase negative staphylococci (CNS) in spent and unspent peritoneal dialysate of physiologic (1.25 mmol/l) and standard (1.62 mmol/l) [Ca2++] was studied. Dialysate was prepared by filter sterilization and pH and iCa were measured. Three isolates of CNS grown in unsupplemented Mueller Hinton broth for 18 to 20 hr at 37 degrees C were adjusted to a density of 1.5 x 10(8) CFU/ml. The dialysate (5.4 ml) was inoculated with 0.6 ml of a 10(-4) dilution and then incubated aerobically at 35 degrees C. Plates were made at 0, 6, and 24 hr, and CFU/100 microliters were counted after 24 hr. In a larger study, 10 isolates of CNS were examined. Coagulase negative staphylococcal growth was inhibited by unspent dialysate (P = 0.000; unpaired t-test). Growth was greater in spent physiologic [Ca++] dialysate (iCa -0.76 mmol/l) at 6 hr (P = 0.007) and at 24 hr (P = 0.000) than in standard dialysate (iCa = 0.86 mmol/l) for two of three strains in the pilot study. Similar findings were noted for 4 of 8 strains in the larger study. There was an overall trend for greater growth in physiologic [Ca++] dialysate (P = 0.09). Some strains of CNS grow better in lower [Ca++] dialysate, perhaps representing clinically relevant trophism.