The objective of this experiment was to test a hypothesis that L-carnitine supplemented diet would improve semen characteristics in 6 adult crossbred AI boars (Hampshire x Pietrain). The control and the tested diet were identical except the tested period (8 weeks) when the diet was supplemented with 2 g of L-carnitine per boar per day. The semen was collected regularly weekly by a gloved-hand technique. Semen volume, sperm motility and concentration, proportion of alive sperm cells and aspartate aminotransferase (AspAT) activity of semen were determined immediately after the semen collection once every two weeks. Sperm survival rate, morphologically abnormal spermatozoa, seminal plasma mineral components and free amino acid concentration, L-carnitine concentration in semen plasma and in sperm cells were determined after the sample storage (-20 degrees C) at a later time.