In New Zealand, crossbreeding is largely used by dairy farmers. Currently an important proportion of cows are crossbreds, mostly Holstein-Friesians (HF) x Jersey (JE). Crossbred bulls are currently being progeny tested in New Zealand. Actually, more than one third of the replacement dairy heifers are crossbred animals (Montgomerie, 2002). However currently available methods to model genetic contributions of purebreds to crossbreds take breed differences only partly into account and therefore do not permit an optimal use of crossbred data. The first objective of our study was to allow the modelling of different additive breeding values according to parental breeds to define overall additive breeding values as a function of breed composition.