Although calmodulin is generally regarded as a soluble protein, a considerable amount of calmodulin activity was found to be associated with particulate fractions of mammalian tissues after an extensive washing of the particulate fraction with EGTA. Identity of this particle-bound and EGTA-nonextractable form of calmodulin with soluble calmodulin was established recently (Sobue, K., Yamazaki, R., Yasuda, S., & Kakiuchi, S. (1981) FEBS Lett. 129, 215-219). The particle-associated calmodulin activity was latent to some extent and its unmasking required the presence of nonionic detergent. We have developed an assay method for the soluble and particulate forms of calmodulin in biological samples and, by means of this method, concentrations of calmodulin in rat and bovine tissues were quantitatively determined. In the supernatant, high levels (greater than 10 microM) of calmodulin were found in the testis, pituitary gland, and various areas of brain, intermediate levels (5-10 microM) in the liver, kidney, and spleen. Particulate fractions contained 10-50% of the total calmodulin contents in the tissues. Human erythrocytes contained (2.5 +/- 0.2) microM calmodulin, or (14 +/- 0.9) X 10(4) calmodulin molecules per cell.