After the Fukushima Daiichi Nuclear Power Plant (FNPP), biological and chemical methods have been developed to reduce radioactive contamination. We hope to know efficient technology to remove radioactive cesium (Cs) from animal and human bodies to avoid internal exposure to contaminated water and foods. It was shown that common enteric bacteria, such as genus Bacteroides and Clostridium, were able to trap cesium-137 (137Cs) in BHI medium (the uptake ratio 45.0–81.2% in vitro). When potassium ion (K+) increased in the medium, % uptake decreased (21.0–24.5%). Viable bacteria are essential because heat-killed bacteria cannot act adsorptive agent. Bifidobacterium longum (intestinal and probiotic bacterium) could grow in BHI (K ion: 2 × 102 ppm) but not in lactobacilli medium. The uptake ratio of B. longum was 37.8% in BHI medium. Lactobacilli, as probiotic bacteria, strongly require K+ ion and can grow in MRS medium with high K+ concentration (1.5 × 103 ppm). It was suspected that K+ inhibits the uptake of 137Cs in MRS medium. Lactobacilli could grow in skim milk medium (5% skim milk, K ion: 7 × 102 ppm), and the uptake of 137Cs increased from 28.7% to 35.7% in Lactobacillus gasseri, from 35.3% to 51.3% in Lactobacillus delbrueckii subsp. bulgaricus and from 33.0% to 82.9% in Lacticaseibacillus casei. Thus, 137Cs uptake may depend on the concentration of K+ in the media under suitable bacterial growth. It was considered that intestinal bacteria are important to remove radioactive Cs from human and animal bodies, although intestinal flora itself can change with aging, diet, and various factors. We also described the possibility of probiotic bacteria for the elimination of radioactive Cs.