Abstract Millipore membrane diffusion chambers containing activated oncospheres of T. hydatigena, T. ovis or T. pisiformis were implanted intraperitoneally (i/p) into rabbits. T. pisiformis and T. hydatigena developed to larvae 3 mm in length within diffusion chambers, but T. ovis oncospheres underwent little reorganisation. Exogenous and possibly somatic antigens released by T. pisiformis larvae developing in diffusion chambers stimulated strong immunity in rabbits to challenge infection with T. pisiformis eggs. T. hydatigena induced a high degree of cross immunity against T. pisiformis larvae following either oral dosing with eggs or development of larvae in i/p diffusion chambers. T. ovis failed to cross immunise against T. pisiformis infection by any of the methods studied. Oncospheres of all three species hatched and activated in the small intestine of rabbits and readily penetrated the intestinal epithelium and passed to the liver. Examination of histological sections suggested that oncospheres primarily forced their way between epithelial cells rather than utilizing lytic enzymes to destroy them. T. pisiformis larvae followed their normal pattern of development in the liver. Both T. hydatigena and T. ovis produced liver lesions identical to those of T. pisiformis at 2 and 4 days after infection (DAI). Lesions due to T. ovis had resolved by 8 DAI and those produced by T. hydatigena by 16 DAI. Although T. hydatigena had not undergone any apparent advanced development in the rabbit's livers, antigens had been released which cross immunised against T. pisiformis ; these antigens must, therefore, be produced at an early stage in larval development. Rabbits vaccinated with antigens collected during in vitro cultivation of T. ovis larvae showed significant enhancement of the T. pisiformis challenge infection. The possible significance of cross immunity in relation to the frequency of occurrence of harmful taeniid infections of man is discussed.