Pseudomonas aeruginosa strain PAF41 was found to produce a new F-type pyocin, pyocin F3, the action spectrum of which was different from those of previously reported pyocins F1 and F2. These three F-type pyocins were compared with respect to their structure and biological properties. These pyocins were almost the same with regard to the structure and the dimensions, and have similar amino acid compositions and S values. The particle weights of these pyocins were also suggested to be similar. Analyses of subunit proteins by SDS-polyacrylamide slab gel electrophoresis showed that these pyocins were composed of 5 major (bands 1, 2, 3, 4, and 6) and 2 minor (bands 5 and 7) subunit proteins and that no difference in the mobilities of these subunit proteins could be detected among the pyocins except that of the second major subunit protein (band 4), which did differ. Pyocins F1, F2, and F3 were immunologically cross-reactive, and carried common antigens as well as specific ones. It was shown that band 6 was a common antigen among the three pyocins and that band 4 was antigenically different in pyocins F1 and F3 by immunological reaction after protein blotting. Electron microscopic observation of pyocin particles treated with anti-sera revealed that the common antigens were located on the rod part and the specific ones were on the fiber part. Pyocin F3 was neutralized by both anti-F3 and anti-F1 sera showing apparent first order rate kinetics, whereas the neutralization for pyocin F1 by these sera did not show such kinetics, but a considerable increment of pyocin F1 activity was observed when small amounts of the sera were added. The increment seemed to be due to the antibodies common to pyocins F1, F2, and F3. A phage, which had a flexuous rod-like tail, was found to be immunologically cross-reactive with the three pyocins and was named KF1.