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ExtractAn antiserum agar medium was-evaluated to determine its usefulness and accuracy in the study of the prevalence of Haemophilus influenzae type b ( H I B ) in the pharynx of infants and young children. The effects of varying the concentration of ingredients (Levinthal base, antiserum, and bacitracin) were demonstrated. The medium could be stored at 4' for periods up to 12 weeks with no loss of effectiveness. All halo-producing organisms detected after 24-40 hr of incubation were found to be HIB. When fewer than 20 colonies of H I B were.found on antiserum agar, H I B could not be detected on either rabbit blood agar or chocolate human blood agar simultaneously inoculated with throat swab fluids. Radial streaking with a 0.001-ml calibrated loop yielded at least twice as many isolated colonies with halos than did inoculation by streaking from a children and is rarely found in adults (16). However, these estimatcs of frequency are based on relatively small numbers chiefly because of the time-consuming bacteriologic differentiation of H I B from other organisms in the pharynx, particularly the much more prevalent nonencapsulated H . i n f l u e n z a e , H . influenzae of other capsular types, H a e m o p h i l u s parainfluenzae and H a e m o p h i l u s h e m o l v t i c u s .Recently, an antiserum agar technique originally described by Petrie (I I) and later used by Pittman (12) and her associates and by Ouchterlony (lo), was utilized by Bradshaw e t a!. (2) to study bacterial antigens cross-reactive with the capsular polysaccharide of H I B in children and in laboratory animals. In the present report, an antiserum agar medium was evaluated to determine its usefulness and accuracy in the study of the prevalence of HIB in infants and young children. swabbed area of the plate.Oropharyngeal culture of 543 healthy Pittsburgh preschoolchildren indicated that the prevalence of H I B is about 3 % in This technique utilized hyperimmune HIB antiserum and bacichildren age 6 months through 3 years; H I B colonization was not tracin incorporated into clear agar with a Levinthal base. Identifiobserved during the first 6 months of life. There was a much higher cation was achieved by observing halos of precipitation surroundcarrier rate (65% ) for siblings of patients with H I B meningitis or ing individual colonies of HIB. epiglottitis, as well a s a greater intensity of colonization; nearly HIB antisera was prepared in goats, sheep, and burros as three-quarters of the sibling carriers had over 5 x lo4 HIB described previously (13). The concentration of precipitating colonies/throat swab. anti-type b antibody ranged from 1.8-3.4 mg protein/ml. LevinThe medium is dependent on large amounts of potent and highly thal base with added N A D (17) at 1.0 wg/ml was stored at -Zoo. specific antiserum, a concentrated Levinthal base, and a refined Bacitracin (]8), brain heart infusion (BHI) (19), and SeaKem agar. Its advantages are rapidity, sensitivity, specificity, and a agarose (20) were used without modification. potential for q...