A novel method based on mathematical analysis of the dose-response curve obtained by radioimmunoassay was used to determine anti-HBs affinity and concentration in 29 healthy subjects vaccinated with plasma-derived hepatitis B vaccine. Anti-HBs affinity as well as its serum concentration increased with time in most vaccine recipients; however, the pattern of affinity maturation varied widely between individual subjects. In 20 subjects the highest increase in antibody affinity was observed after the third dose of vaccine, whereas in nine cases values of affinity remained low and relatively constant throughout the time of the study.