Summary -Responses to immunization with aggregated human serum albumin (HSA) and to repeated experimental infections with H contortus were studied in 51 female lambs of the Romanov breed, born from 8 sires and 36 dams. The 8 sires were of haemoglobin genotype Hb AB; the 51 lambs were distributed into 3 groups of 17 each, corresponding to the 3 genotypes HbAA, HbAB and HbBB. In addition the experimental lambs were typed for antigens of the major histocompatibility system (OL./1). The