Ninety-eight hospital strains of Pseudomonas aeruginosa isolated from six different hospitals in Athens were serotyped by a slide agglutination test with unabsorbed commercial antisera. Serotypes 06, 011, 012, and "pool E" strains (strains that agglutinated only in pool E, which contained antisera against 02, 05, 015, and 016 antigens, but did not agglutinate in the individual antisera) predominated, accounting for more than 62% of all isolates tested. In respect to serotypes, (i) there was no apparent correlation with hospital of origin, (ii) most strains of serotypes 06 and 011 were sensitive to gentamicin and carbenicillin (iii) most strains of pool E were from urine and were resistant to these drugs, (iv) all 9 strains of serotype 012 tested were resistant to carbenicillin and all 5 strains tested hydrolyzed this drug, and (v) 24 of 25 strains of pool E were resistant to carbenicillin but only 2 of 17 strains hydrolyzed it.