Poly(A) signals of mammalian pre-mRNA have been defined as an AAUAAA sequence 10-30 nt upstream of the cleavage/poly(A) site followed by a GU/U-rich element immediately downstream. However, a number of viral poly(A) signals have been shown to possess additional signals upstream of AAUAAA that increase poly(A) site efficiency. We describe the first non-viral example of such an upstream sequence element (USE) for the poly(A) site of the human C2 complement gene. As this gene is very closely spaced to the related Factor B gene [the C2 poly(A) site is only 421 bp from the transcription start site of Factor B] we have isolated this same intergenic sequence from four other mammals (mouse, cat, rabbit and cow). We show that the USE of the C2 poly(A) site is highly conserved between these five different mammals. Furthermore, extensive mutagenesis of the human USE indicates that most of the 53 nt sequence is required for full activity. The human C2 poly(A) site does not possess any obvious downstream GU/U-rich sequences, although sequences immediately 3' to AAUAAA as well as 13 nt of sequence following the cleavage site are both required for full activity. Interestingly the other mammalian C2 poly(A) sites do possess significant downstream GU/U-rich sequences. Finally we show that all five mammalian C2 poly(A) signals are immediately followed by conserved signals for transcriptional termination, consistent with the close proximity of the downstream Factor B gene.