Eight subjects with end-stage renal disease were given gentamicin alone (single dose) and in combination with ticarcillin (multiple doses) to determine to what extent ticarcillin inactivates gentamicin in vivo and when this inactivation becomes significant. Gentamicin serum concentrations were performed by both a rapid radioimmunoassay (1 1/2 hours) and a conventional bioassay (8 hours) to determine whether the bioassay results would be falsely lowered by ticarcillin. The results of the study showed that ticarcillin inactivates gentamicin in vivo with an inactivation rate of 0.01 hours(-1). This was demonstrated by a decrease in serum gentamicin levels which became significant at 12 hours after gentamicin was given, and a decrease in gentamicin half-life from 46 to 28 hours during ticarcillin administration. Ticarcillin did not interfere with the bioassay method of measuring gentamicin levels. Although ticarcillin inactivates gentamicin in subjects with end-stage renal disease, the serum level data of gentamicin suggest that supplemental doses of the aminoglycoside are not necessary for 48 hours after a single loading dose of gentamicin. Widely used microbiological assay procedures are reliable in determining gentamicin levels and do not appear to be falsely lowered by concurrent ticarcillin administration.