We recently showed that Helicobacter pylori grown on plates produce cecropin-like antibacterial peptides to which H. pylori is resistant. This antibacterial activity was traced to fragments from the N-terminus of ribosomal protein L1 (Pütsep et al., Nature, April 22, 1999). The evolutionary suggestion from this finding has now been extended by the synthesis of eight peptides with sequences taken from the N-terminus of ribosomal protein L1 (RpL1) of five different species. Two peptides of different length derived from H. pylori RpL1 showed a potent antibacterial activity, while a peptide with the sequence from Escherichia coli was 20 times less active. Like cecropins the H. pylori peptides were not cytolytic. We suggest that the cecropins have evolved from ribosomal protein L1 of an ancestral intracellular pathogen that developed to a symbiont ending as an organelle. When the R1 gene moved into the host nucleus, a duplication provided a copy from which today cecropins could have evolved.