Antibodies to the snRNP proteins B' and B are frequently observed in autoimmune diseases. We analyzed different types of cDNAs encoding these proteins. One type of cDNA encoded a protein whose predicted mol. wt is 24.6 kd, whereas another type encoded a protein with a predicted mol wt of 23.7 kd. When translated in vitro from cDNA transcripts, the apparent mol. wts of these proteins on SDS-polyacrylamide gel were 29.5 and 28.5 kd respectively. The main difference between these two types of cDNAs proved to be the presence of an additional sequence of 146 nucleotides in the 3' part of the open reading frame of the clone encoding the shorter protein. This insert contains a termination codon in frame, at 9 nucleotides downstream from the 5' end of the insert. The additional sequence revealed at the 3' end a consensus sequence of vertebrates for intron-exon junctions. We demonstrated the presence of mRNAs corresponding with both types of cDNA in human cells. We hypothesize that the B' and B protein are derived from one pre-mRNA by alternative splicing, and show that they differ only at the carboxy terminus, where a proline rich motive is repeated once more in B'. A comparison with amino acid sequences of other cloned snRNP proteins is included.