OBJECTIVE To determine if anti-idiotype antibodies and circulating immune complexes in individuals before and after immunisation with tetanus toxoid play a role in the immune response. METHODS A study of individuals who were administered a single dose of tetanus toxoid (TT) and who were unimmunized. METHODS Out patient departments of a large public hospital in Bombay, India. METHODS Thirty eight individuals pre-immunisation and forty five individuals post-immunisation with tetanus toxoid, tested at 1, 3, 6, and 12 months. METHODS Development of anti-tetanus anti-idiotype antibodies and circulating immune complexes. RESULTS Pre-immunisation cases did show presence of anti-tetanus antibodies but in lower titres than post-immunisation up to six months, after which there was a reduction. Specific anti-idiotype antibodies were detected in 19 cases. One and three months after immunisation more cases had high titre antibodies and circulating immune complexes, though after six months, there was a fall in anti-tetanus antibody titres. Circulating immune complexes were seen in those samples having anti-idiotype antibodies. CONCLUSIONS Though a significant rise in anti-tetanus antibody anti-idiotype antibodies, protective levels in mice and circulating immune complexes are seen after immunisation with TT it lasts for six months. When followed up for a period of one year it is observed that in cases having auto anti-idiotype antibodies, the anti-tetanus antibodies are maintained for a longer period.