An intracellular selection system allows screening for peptide bioactivity
and stability. Randomized recombinant peptides are screened for
bioactivity in a tightly regulated expression system, preferably derived
from the wild-type lac operon. Bioactive peptides thus identified are
inherently protease- and peptidase-resistant. Also provided are bioactive
peptides stabilized by a stabilizing group at the N-terminus, the
C-terminus, or both. The stabilizing group can be a small stable protein,
such as the Rop protein, glutathione sulfotransferase, thioredoxin,
maltose binding protein, or glutathione reductase, an .alpha.-helical
moiety, or one or more proline residues.