In order to study the antigenic properties of the La protein we have isolated a 1650 base pair (bp)-long human cDNA encoding an anti-La reactive protein. Restriction enzyme analysis and DNA sequencing was used to compare this clone with two published but inconsistent partial sequences. Our clone extends about 220 bp further towards the 5' end than the two clones previously studied and includes a putative initiation codon. When introduced into an expression vector, stable fusion proteins were made both from the initial clone and from two deletion clones. The recombinant proteins were tested by immunoblotting against a panel of anti-La sera. All reacted with the fusion protein produced by the 1650-bp clone. About half of the anti-La sera showed reactivity against the recombinant protein from the shortest deletion clone. This indicates the presence of an epitope in the amino terminal part of the La protein, encoded by sequences not present in previously published clones.