The genetic contribution of the Friesian, Holstein, Ayrshire and Finncattle breeds to the Finnish Black and White cattle was estimated for animals born between 1976 and 1991. Pedigrees of 219 422 cows and 1019 bulls were traced back for a maximum of 10 generations. Based on the results for the genetic constitution, cows were grouped as (i) pure Friesian cows, (ii) cows with 25% Holstein genes and (iii) cows with 50% Holstein genes. The estimated breeding values for protein yield, days open and somatic cell count were used to investigate the effect of increasing Holstein blood in the Finnish Black and White cattle population. The results showed that the proportion of Holstein genes in the Black and White cows increased from 0.2% in 1976 to 33.3% in 1991. The proportions of genes due to the contributions of Ayrshire and Finncattle decreased from 8.8% and 3.0% in 1976 to 6.0% and 0.4% in 1991, respectively. The Holsteinization caused only a small increase in protein yield but a considerably higher increase in days open. The variation in EBVs for somatic cell count was somewhat smaller for cows with 25% or 50% Holstein genes.