Whole cells, cytoplasms and peptidoglycans of ten different lactic acid bacteria (LAB) were tested for in vitro cytotoxicity on diverse cancer cell lines using the H-3-thymidine incorporation assay. The peptidoglycans and cytoplasm fractions, as well as heat-killed whole cells of LAB, had significant antiproliferative activities against several cancer cell lines. In particular, the cytoplasm fractions exhibited marked direct antiproliferative activities against colon and gastric cancer cell lines, whereas the peptidoglycans retarded growth of colon and bladder cancer cell lines. The cytoplasm fractions of Bifidobacterium longum and Lactococcus lactis ssp. lactis inhibited proliferation of two cancer cell lines by 50% at 33 and 23 mug ml(-1) for SNUC2A (a human colon adenocarcinoma cell line) and 17 and 11 mug ml(-1) for SNU-1 (a human gastric cancer cell line), respectively.