Enthusiastic formulation of clinical guidelines continues to increase but although theoretical difficulties in guideline implementation have been recognised, little attention has been paid to their effectiveness in everyday clinical practice. The introduction of a protocol for empirical treatment of lower respiratory tract infection (PETRI) to an acute medical take-in unit in Belfast is described. Early involvement of all relevant staff, preparation of user-friendly flow charts, and imaginative publicity, resulted in an initial implementation rate of 75%. The role of implementation as a significant rate-limiting step in the audit cycle is emphasised.