SummaryIn a multicenter study the susceptibilities of 1748 Gram-negative bacilli, isolated from hospitalized patients, have been tested to piperacillin in comparison with other betalactam antibiotics: 75 % of theEnterobacteriaceaeand 79 % of thePseudomonas aeruginosaisolates were found susceptible.The percentages inhibited by ticarcillin were 62 % and 72 %, and by gentamicin 94 % and 75 % respectively.