The need for p-aminobenzoic acid of a previously reported pab mutant in a Salmonella typhi strain causes loss of virulence (mouse median lethal dose by the intraperitoneal route with mucin, ca. 10(7) CFU, versus less than 200 CFU for related pab+ strains). This strain, however, gave p-aminobenzoic acid-independent revertants at a low frequency (ca. 4 X 10(-10) per bacterium per generation). It was therefore given, by transduction and mutation, a transposon-generated, nonreverting (rate, less than 3 X 10(-11) per bacterium per generation) mutation at purA, causing a requirement for adenine; such a mutation in a wild-type strain caused about the same loss of virulence as the pab mutation. The pab purA strain, 205aTy, has mouse median lethal dose of ca. 5 X 10(7) and is expected to be unable to cause typhoid fever. Since strain 205aTy behaves like a typical Vi-positive S. typhi strain in nearly all common tests, we propose that it is a safe strain for use in laboratory teaching, proficiency testing, and the like.