The introduction of Haemophilus Influenzae type b (Hib) polysaccharide vaccine in 1985 provided an opportunity to study the extent to which physicians have adopted the use of the new vaccine in their routine practice behavior, the factors that predict Hib vaccine use by physicians, and how physicians have chosen to deal with differing recommendations for its administration. We surveyed all physicians providing primary care to children in New Mexico six to eight months after vaccine licensure to assess their knowledge of existing recommendations and their current behavior regarding use of the Hib vaccine. Of the 369 primary care physicians who responded, 100% of pediatricians, 98% of family practitioners, and 91% of general practitioners were aware that a vaccine against invasive Hib disease had been licensed. Sixty-three percent of physicians surveyed were currently using the vaccine. Vaccine usage varied significantly by specialty, with 86% of pediatricians reporting use compared with 61% of family practitioners and 31% of general practitioners. Physicians were significantly more likely to use Hib vaccine if they were young and if they worked in the private sector rather than the public sector. The three sets of recommendations for Hib vaccine use had been widely read; 85% of respondents had read at least one set. Practitioners confronted with differing recommendations had elected to give the vaccine permissively within existing guidelines. Our findings suggest that practitioners have adapted quickly to the introduction of Hib vaccine but that nonuse of the vaccine in the public sector remains an obstacle to full implementation of this prevention strategy.