In hospitals in Spain there has been a large increase in the use of wide spectrum antibiotics. The objective of this study was to evaluate the influence of a rational, consensual protocol on the use of ceftriaxone. An observational and prospective study was carried out using information provided by the Servicio de Farmacia on the in-hospital prescriptions for ceftriaxone. The study included two groups as follows: a control group (October-December 1995) not taking into account the protocols for antibiotic treatment; and a study group in the same months in 1996 with the consensual protocols taken into account. The criteria for the type of patient, infection, evolution and antibiotic treatment were evaluated according to the criteria in the international literature on this type of study. The initial characteristics of both groups of patients were similar. Following the use of the protocol there was a significantly associated reduction in use (48 cases in the study group vs. 57 cases in the control group), an increase in the appropriate use (31. 57% in the control group vs. 75% in the study group), an increase in the cure rate (70.17% in the control group vs. 91.7% in the study group) and a decrease in hospital stay of 7.53 days (22.59 days in the control group vs. 15.06 in the study group), producing a savings per cured patient of 617.142 pesetas. It was concluded that the consensual protocol and its proper application make for a highly interesting approach given that it allowed for better quality use of ceftriaxone with greater cost-effectiveness.