Ticarcillin disodium and gentamicin sulfate serum levels were measured one and five hours after intravenous administration. Patients receiving ticarcillin plus gentamicin had a significantly lower mean gentamicin level one and five hours after antibiotic administration than patients receiving cephalothin sodium plus gentamicin. The serum ticarcillin levels were significantly lower five hours after administration in patients receiving ticarcillin plus gentamicin than in those receiving ticarcillin plus cephalothin. Low antibiotic serum levels in patients with serious infections treated with these antibiotic combinations are of potential clinical significance; therefore, to ensure an optimal antimicrobial therapy, it is advisable to determine the drug serum concentrations (especially aminoglycoside) even in patients with normal renal function when treating with combinations of antimicrobials.