Print field with an array
Assuming this is an entity (it looks like it probably is), you might want to look into using entity metadata wrappers. It'll make your theming life much easier.

Otherwise, to immediately display your field, this would work (assuming a node):

echo $node->[field_serial_571][LANGUAGE_NONE][0]['value'];

LANGUAGE_NONE is a constant for an undefined language, in your example the 'und' part of the array. It's best practice to use that constant rather than using 'und'.

Read even more about entity metadata wrappers here: http://www.pixelite.co.nz/article/how-use-entity-metadata-wrappers-drupal-7/