Hereditary cystatin C amyloid angiopathy (HCCAA) is an autosomal dominant condition in which the patients suffer at an early age from repeated cerebral haemorrhages. The development of HCCAA is directly linked to a Leu-68-->Gln (L68Q) mutation in the cystatin C protein sequence. The concentration of cystatin C in cerebrospinal fluid (CSF) of HCCAA patients is markedly diminished and cultivated monocytes from affected individuals accumulate cystatin C. The goal of this work was to characterize cystatin C isolated from CSF and monocyte cultures originating from healthy persons and HCCAA patients with respect to the L68Q mutation. Cystatin C was isolated by carboxymethylpapain affinity chromatography. Proteins from CSF and monocyte cultures that bound specifically to the carboxymethylated papain column were resolved by reverse-phase HPLC chromatography and tryptic peptides were subsequently analysed by matrix-assisted laser desorption ionization MS. No evidence for mutated cystatin C protein was found in CSF samples from healthy subjects or HCCAA patients, but approx. 60% of the protein was found to be hydroxylated on Pro-3. No evidence was found for secretion of mutated cystatin C from HCCAA monocytes. However, we obtained evidence for the presence of mutated cystatin C in HCCAA monocytes. These results support the conclusion that the mutated cystatin C is retained in association with the monocytes and not secreted. An increased intracellular concentration would presumably promote the aggregation and denaturation of the mutated cystatin C, leading to the formation of amyloid fibrils and cell death.