A deterministic, compartmental, mathematical model is used to describe the transmission dynamics of hepatitis B virus (HBV) in a high endemicity country. All three major transmission routes are included in the model, that is, perinatal, horizontal and sexual transmission. The model also reflects the demography of a typical developing country, and incorporates age-dependence in the rates of transmission and the probability of becoming a chronic carrier. Numerical simulations of the model are shown to capture the observed age-specific patterns of serological markers. The sensitivity of the model to age-specific heterogeneities and routes of transmission is investigated. The model is used in a preliminary study of the possible implications of mass infant immunization on HBV epidemiology, and the results suggest that eradication of HBV may be achieved by immunizing less than 70 per cent infants, which is relatively low compared to most childhood viral infections. This is due in part to the interaction between changes to the average age at infection under immunization, and the nature of age-dependence in the force of infection and the probability of becoming a carrier. However, numerical results also suggest that eradication may take many decades to be achieved, largely due to the presence of a pool of chronic carriers. Furthermore, there may be a significant lag between the implementation of mass infant immunization and a decline in the incidence of liver cancer.