Cylindrical tubes of mild steel in the as-received and annealed condition were supported along their lowest generator and perforated normally at their central section by square-section punches possessing pointed pyramidal heads of semi-angles 15°, 22 1 2° and 30° or by conically-headed cylindrical punches with semi-angles of 15° and 30°. The test equipment and procedure are briefly described and results are presented showing typical courses of deformation for both pierced and partially plugged specimens and punch load-displacement diagrams. The distinctly different characteristics of petalling and plugging modes of deformation are discussed and related to punch semi-angle. Interesting features of tube-piercing are compared with the collective peculiarities of both plate-piercing and hole-flanging. For the square-section punches, the effect of punch orientation to the generator is investigated. The effect of tube length on the mode of deformation of the tubes is also examined. For relatively short tubes, calculated theoretical plastic collapse loads agree well with those obtained in the experiments. For long tubes (L > 2·5D) the maximum penetrating load is nearly constant and about the same as that for a length of tube equal to 2 1 2 times the tube diameter.