The effect of previous oral administration ofLactobacillus caseiused as immunomodulator on the inhibition of a fibrosarcoma (a non‐intestinal tumour) was studied. BALB/c mice were fed with different doses ofL. casei(2.4, 6 and 8.4 ×109bacteria). At the end of each treatment, 5 ×105tumour cells from a methylcholanthrene‐induced fibrosarcoma were subcutaneously inoculated, tumour progression being studied for 35 days. Results showed that feeding with a 2.4 ×109dose ofL. caseibefore inoculation of the tumour cells caused a 41% suppression in tumour growth. In the case of those animals fed with the same dose in which there was no inhibition of tumour growth, an increase of 55 days in survival time was observed as against a control survival period of only 35 days. The other doses assayed proved unable to check tumour growth; on the contrary, they favoured it.