Streptomyces lividans IAF18, obtained by homologous cloning, is capable of over-producing XlnA. To investigate the possibility of the expression of foreign genes, various coding regions of the xylanase A gene (xlnA) were analysed. Expression/secretion vectors were constructed containing the regulatory elements of xlnA with the coding region of the leader peptide with or without the truncated structural gene encoding the first 310 amino acids of the XlnA. The genes coding for the Escherichia coli beta-glucuronidase and subunit 1 of the Bordetella pertussis toxin (S1) were used and their expression analysed, S. lividans transformants where the beta-glucuronidase gene was fused with the leader sequence produced up to 30 mg beta-glucuronidase/culture filtrate whereas only fused XLnA/S1 was detected and its yield was estimated to be 1 mg/l. The disappearence of the B. pertussis toxin S1 and beta-glucuronidase from the culture medium was due to the concomitant appearence of secreted proteases from S, lividans.