A study of poliomyelitis neutralizing antibodies was carried out on 165 samples of blood serum drawn from subjects aged 14-15 years who had completed vaccination with OPV 11 or 12 years ago. Two different methods of incubation of the serum-virus mixtures (1 hour at 37 degrees C; 6 hours at 37 degrees C and then 18 hours at 4 degrees C, respectively) and two different cell lines (RC-37 and Hep-2) for the inoculation of these mixtures were employed. The results were also evaluated in relation to different initial dilutions of the sera (from 1:4 to 1:1). With the 1:4 initial serum dilution the highest frequencies of sero-negativity were observed utilizing the short incubation time and inoculation in Hep-2 (42% for polio 1, 10% for polio 2, 32% for polio 3). These frequencies diminish significantly utilizing the RC-37 cell line and the long incubation respectively. Under all the experimental conditions the results were better when the 1:2 initial serum dilutions were employed and even better when undiluted sera were used. Neutralizing antibodies against the 3 types of poliovirus were detected in the undiluted sera of all the subjects utilizing the long incubation and inoculation in either the RC-37 or the Hep-2 cell lines. In three subjects lacking detectable antibodies at the 1:4 dilution of the sera, the administration of a dose of IPV produced after 5-7 days a high response which remained almost unchanged 30 days later.