Sampling conditions are of utmost importance for the determination of trace elements in biological fluids. However, many biologists still underevaluate this critical step. This reports deals with collection of biological fluids from a routine point of view: detailed and careful descriptions of the contaminating role of air and the general handling of specimens are given in a practical and critical manner. The main commercial systems, made about needles, catheters, tubes (open and evacuated systems) and anticoagulants are reviewed. Choices and elementary rules, applicable in a hospital setting, are proposed.