The viral DNAs induced by the unclassified animal virus, chicken anaemia agent (CAA), during replication in MDCC-MSB 1 cells have been investigated. Analyses after S1 nuclease, restriction endonuclease and denaturation treatments indicated that infected cell extracts contained genome-size, single-stranded DNA (M(r) 2.3 kb), closed and open circular, double-stranded replicative form (RF) DNAs (M(r) 2.3 kbp) and a population of smaller double-stranded DNAs (M(r) 0.8 kbp). Recombinant plasmids containing 2.3 kbp CAA RF fragments cloned at the PstI, BamHI and EcoRI sites failed to transfect MDCC-MSB 1 cells. However, one plasmid, which contained two 2.3 kbp CAA RF fragments ligated in tandem at the PstI site, and cloned 2.3 kbp PstI, BamHI and EcoRI fragments, excised from their respective plasmids by restriction endonuclease digestion, were capable of transfection. The nucleotide sequence of the circular genome (2298 bp) of the Cux-1 isolate of CAA has indicated the presence of three overlapping open reading frames (ORFs) of 52 kDa, 24 kDa and 13 kDa on one strand. The existence of these ORFs was corroborated by analyses of partial sequences from three other isolates. The non-coding region of the CAA genome contained sequences with putative regulatory function. These results are discussed in relation to the "rolling circle" model of DNA replication.