Employed since 1971, contra-immunoelectrophoresis is a microbiological technique now recognised as being of increasing value. Used in 37 cases of pneumococcal lung infections its diagnostic reliability was 81% the technique being of particular interest in that it enabled a positive diagnosis in lung infections previously treated by antibiotics. Though performed in only a small number of cases, this method was able to confirm the frequency of pneumococcal infections. The technique can now be considered to be well established, the production of specific anti-serums, approaching type-specificity, being possible. Fairly simple to perform, and giving rapid results (within hour) it requires a minimum of apparatus and is a standard procedure that should be employed in all microbiological medical laboratories. Of undeniable diagnostic value, it should be associated with conventional bacteriological tests, which though essential sometimes give insufficient data.