It has been known for some time that the micro-milieu of solid tumours provides an ideal environment for growth of facultative and strictly anaerobic bacteria, and it has been shown that certain species including Lactobacillus and Clostridium can colonise those environments leading to regression of tumour growth. Such observations have given rise to the concept of bacteriolytic therapy where live microorganisms might be employed to colonise the tumour and exert a tumorolytic effect. In choosing such an approach, it would be advantageous to exploit a relatively non-pathogenic strain and provide some form of containment that would enable site-specific injection and minimise dispersion of the microorganism throughout the host. In testing the feasibility of such an approach, we prepared microencapsulated formulations of Lactobacillus casei NCDO 161 and demonstrated that conditioned extra-capsular culture media were toxic to tumour cells in vitro. We further investigated the effects of the microencapsulated formulations on tumour growth in vivo following direct intra-tumoural injection. The study demonstrates significant inhibition of tumour growth in vivo by these formulations and suggests potential therapeutic benefit of this approach in the treatment of solid tumours.