A cDNA expression library in lambda gt11 prepared from cDNA derived of seminal vesicle tissue was screened by means of monospecific rabbit anti-aSFP IgG. The sequence of clone pTF21, containing an insert of 668 bp comprised an open reading frame from position 7 to 411 terminated by two stop codons. From this sequence a protein of 134 amino acid residues can be deduced. The mature aSFP was preceded by a signal peptide of 20 amino acids length. The protein sequence contains no signal for N-glycosylation. The molecular weight calculated from the amino acid sequence is 12922 Da. The start codon ATG is part of the sequence AAGATGA which fulfills the criteria of an initiation consensus sequence. The coding region was followed by 257bp of the complete 3'-untranslated region (3'UTR). A putative polyadenylation signal AATAAT, although not of the standard type, is observed at position 650. According to Northern analysis, aSFP mRNA is expressed in seminal vesicle tissue, ampulla and weakly in tissue of epididymis, but not in testis or other bovine tissue. aSFP is specified by a single copy gene. Attempts to detect homologies to known protein sequences were not successful.