The effect of yoghurt and soya yoghurt containing Bifidobacterium lactis Bb-12 or B. longum Bb-46 on Ehrlich ascites tumour cell proliferation was investigated in vitro and in vivo. Tumour cells were incubated with B. lactis Bb-12 or B. longum Bb-46 cultivated in de Mann Rogosa Sharpe (MRS) broth medium, or with their centrifuged supernatant fractions or sediments, for 2 h at 37°C. Treatment resulted in the inhibition of tumour cell proliferation by 85·42 (sd 0·78) and 85·10 (sd 1·28) % by intact micro-organisms, 77·61 (sd 0·29) and 71·43 (sd 1·75) % by their supernatant fractions, but only 4·00 (sd 0·19) and 9·09 (sd 1·24) % by the two sedimented bacteria, respectively. The incubation of tumour cells with yoghurt and soya yoghurt containing Bb-12 for 2 h resulted in 83·01 (sd 0·11) and 88·23 (sd 0·06) % inhibition, respectively, while it was 83·82 (sd 0·24) and 86·36 (sd 0·06) %, respectively for the same products containing Bb-46. Corresponding values for plain yoghurt and soya milk (without bifidobacteria) were 32·81 (sd 0·14) and 5·55 (sd 0·12) %, respectively. The differences between yoghurt or soya yoghurt containing Bb-12 or Bb-46 and plain yoghurt, soya milk or control treatments were statistically significant (n 3; P < 0·05). Female Swiss albino mice were injected intraperitoneally with the same tumour cells. The lifespan of mice fed diets supplemented with yoghurt or soya yoghurt containing Bb-12 or Bb-46 was prolonged by 16, 23, 34 and 39 %, respectively compared with that of the positive control group (n 6; P < 0·05). The lifespan of groups fed plain yoghurt or soya milk was prolonged by 15 and 8 %, respectively. Prolongation of lifespan was positively correlated with faeces bifidobacterial count in the groups fed yoghurt or soya yoghurt containing bifidobacteria (r 0·917; P < 0·05).