Necator americanus, originally isolated from man, developed completely in infant rabbits (RSG-1). This infection was serially passed in infant rabbits up to the 6th generation without using any immunosuppressant. Two day old infant rabbits demonstrated a high degree of susceptibility to N. americanus. Studies revealed the retention of worms in the intestine for more than 150 days during which egg production was high. In this experimental model the leukocyte, antibody and serum protein responses due to N. americanus infection were monitored. Results showed that eosinophil numbers rose significantly from day 28 to 170. A maximum was observed on day 80 (41%) in rabbits of the fourth generation. Antibodies to N. americanus were detectable in infected rabbits by counterimmunoelectrophoresis using third stage larval antigen (Rabbit strain). Generally, total globulins increased with decrease in albumin. Serum protein increase was associated with alpha-2, beta and gamma globulin components.