Enzyme immunoassay is the basic method for the detection of antibodies to the Human Immunodeficiency Virus (HIV). In spite of the apparent facility of the serological screening, the difficulties are real at all stages of the analysis. Specimen treatment needs defined rules to avoid contamination. The products authorized are listed by the "French Drug Agency". The specificity cannot be less than 99.5%, and the sensitivity must be of 100%. Each biologist have to acquire its own experience of the tests utilised. To the very recent seroconversions, he must define decision criteria for the pursuit or not of the investigations. False positive have been observed for patients with autoimmune pathology, but was usually related to a disparity between kit lots.