Microsomal and mitochondrial fractions were isolated from rabbit colon muscle homogenate by differential centrifugation. The calcium content of these fractions were rather similar. The microsomes were divided into subfractions by gradient centrifugation and tested on glycerol treated colon muscle. Acetylcholine and imidazole elicited an EGTA-sensitive contraction of glycerinated colon muscles in the presence of microsomal subfractions and Mg-ATP. The glycerinated colon muscle constitute a rapid system for qualitative drug tests on Ca-metabolism in subcellular fractions of smooth muscle.