A wild-type strain of Salmonella enterica serotype Gallinarum, lysogenized with P22 sie (superinfection-exclusion defective) was greatly attenuated for newly hatched or 21-day-old chickens. An aroA transductant of the lysogenic strain and an aroA-serC tetracycline-sensitive deletion or deletioninversion mutant of the latter were equally attenuated. Intramuscular administration of the aroA-serC strain to 21-day-old chickens protected them against oral challenge with 10(6) colony forming units of a highly virulent Gallinarum strain (no deaths in the 30 vaccinated chickens versus 14 of 30 in the control group). There was evidence of protection in the contents, mucosa and lymphoid tissue of the alimentary tract, in addition to that which occurred in the liver and spleen.A weak serological response was detected by enzyme-linked immunosorbent assay, indicating that use of such a strain is compatible with serological monitoring and would be a useful adjunct to control schemes for fowl typhoid.