Data from 75 567 ejaculates from 1 417 boars of the breeds Czech Large White and Czech Landrace collected on 23 AI centres between 2000 and 2007 were analyzed. Fixed effects were estimated from a four-trait animal model for semen volume, sperm concentration, motility and percentage of abnormal spermatozoa and from single-trait animal models for the total number of spermatozoa and the number of functional spermatozoa. Both the total number of spermatozoa and the number of functional spermatozoa were highest in winter than in summer. Boar?s age had a strong influence on semen volume, the total number and the functional number of spermatozoa; these traits increased especially in the first phase. Percentage of abnormal spermatozoa increased with age. An interval between collections of 7 to 10 days yielded best values for all semen traits. As semen traits are of direct economic importance for AI centres, it can be expected that breeding value estimation for semen traits will become importan