We have compared endogenous proviruses in DNA of chickens of 11 breeds by means of Southern's technique. Many of the endogenous virus loci found were missing from the genomes of the extensively studied white leghorn chickens, although some of the proviruses and especially ev-1 appeared to be widely spread among different chickens. Most of the proviruses were similar to Rous-associated virus (RAV-O) as judged by the EcoRI digestion patterns. Several genetically different proviruses were found in the DNA of 28 brown leghorns. They contain neither ev-1 provirus in their genome, nor any other one common for all individuals. All four Italian partridge-coloured chickens examined appeared to be free from ALV-related sequences in their DNA showing the possibility of normal life without known endogenous proviruses. Possible causes of inter-breed differences of chicken endogenous proviruses are discussed.