Four experiments are summarized. Initially an attempt was made to modify strain 19 calfhood vaccination so as to eliminate the persistent serological reactions which interfere with eradication programmes. Later the project broadened into a search for an effective method of vaccination that could be applied when required, to all ages of cattle, thus allowing calfhood vaccination to be safely stopped. In the first experiment, reducing the age at vaccination with strain 19 to 1 month practically eliminated the serological response to vaccination, but the resulting immunity was not satisfactory. However, vaccination at 1 month followed by a booster consisting of a reduced dose of strain 19, given conjunctivally 1 year later, stimulated an immunity at least equal to that given by conventional calfhood vaccination. The vaccination of pregnant cows with either of 2 reduced dose levels of strain 19 gave better immunity than calfhood vaccination with the full dose. Uterine strain 19 infections were unacceptably frequent in cows given 6 X 10(9) c.f.u. of strain 19 in early pregnancy, but no such infections were found in 9 cows given 3 X 10(8) c.f.u. Vaccinal antibody titres declined rapidly in the latter group. Vaccination of mature, non-pregnant heifers with 3 X 10(8) c.f.u. of strain 19 produced immunity at least as good as that produced by calfhood vaccination, with a serological response greatly reduced in the majority of cattle. However, a small proportion of vaccination cattle developed high titres persisting for at least 7 months.(ABSTRACT TRUNCATED AT 250 WORDS)