A 4-month-old infant with congenital heart disease and sepsis and arthritis, and subsequently meningitis, caused by an antibiotic-resistant strain of Haemophilus influenzae type b, failed to respond to sequential therapy with ampicillin and trimethoprim/sulfamethoxazole. Following treatment with ceftizoxime, the infant was well for 42 days, until he returned to the hospital and died. A total of 10 Haemophilus influenzae type b isolates, all outer membrane protein subtype 51, was isolated from the pretreatment blood and synovium, cerebrospinal fluid and subdural fluids, and the petrous pyramids at autopsy. Pretreatment isolates had no detectable plasmid DNA, chloramphenicol acetyltransferase or beta-lactamase; the minimal inhibitory concentration for ampicillin (AM) and chloramphenicol (CM) was 0.2 and 0.8 microgram/ml, respectively. However, all cerebrospinal fluid isolates had a 42-44 mD plasmid and produced chloramphenicol acetyltransferase and beta-lactamase; the minimal inhibitory concentration of these isolates to AM and CM were 12.5 and 25 micrograms/ml, respectively, and were also resistant to tetracycline and sulfonamide. Resistance to AM and CM was cotransferred by filter-mating conjugation at a frequency of one to two transconjugants per 10(5) to an Rd haemophilus recipient. Posttreatment isolates from the petrous pyramids also were resistant to AM and CM and produced chloramphenicol acetyltransferase and beta-lactamase activity, but had no plasmid DNA. These findings and data from genetic studies suggested that plasmid-bearing antibiotic-resistant Haemophilus influenzae type b was selected from a heterogenous population, and that the AM/CM resistance transposons were incorporated into the bacterial chromosome.