The composition and structure of endogenous proviruses present in the genome of Italian Partridge chickens were studied by the method of blot hybridization using RAV-2 [32P]DNA or LTR of RSV as hybridization probes. The genomes of 5 out of 39 chickens analyzed did not contain endogenous proviruses related to RAV-2. Different sets of five so far undescribed endogenous proviruses, differing in the structure and location, were detected in the DNA of other IP chickens. None of them is identical in its structure to the DNA of the endogenous chicken virus RAV-0, all five loci of endogenous proviruses of IP chickens were defective. The origin, the patterns of genetic variation and the function of endogenous proviruses are discussed.