Normal human serum or isolated human amyloid P component was ultracentrifuged on density gradients containing either 10 mM EDTA or different concentrations of Ca2+ between 0.15 and 2.15 mM. In the presence of Ca2+ concentrations of 1 mM or more human P component sedimented more rapidly than it did in the presence of lower Ca2+ levels or of EDTA. This phenomenon was due to Ca2+-dependent aggregation of P component molecules and did not require the presence of any other serum constituents. It was completely inhibited by incorporating a physiological concentration (40 mg/ml) of serum albumin in the gradients, suggesting that free ionized Ca2+ is required to promote aggregation of the P component. P component from the mouse and the plaice (Pleuronectes platessa L.), a marine teleost, did not undergo the same Ca2+-dependent aggregation as human P component. These observations resolve a discrepancy existing in the literature concerning the sedimentation rate of human P component in density gradient ultracentrifugation and shed new light on its behaviour with respect to Ca2+ which may be relevant to the deposition of P component in amyloidosis.