The isolation and analysis of the polysaccharide-peptidoglycan complexes of Lactobacillus casei YIT9018 are presented. Two polysaccharide-peptidoglycan complexes, PS-PG1 and PS-PG2, were solubilized from the heat-killed cell by treatment with N-acetylmuramidase. PS-PG1 was composed of glucose, rhamnose, and small amount of galactose and glucosamine. PS-PG2 was composed of glucose, rhamnose, galactosamine, and glucosamine. The ratio by weight of these fractions was about 1:8. PS-PG2 was analyzed in detail. Smith degradation and deamination of this complex yielded oligosaccharide units. The results of methylation analysis of these units and intact PS-PG2 led to the most probable structure of PS-PG2: (formula; see text)