Abstract It was previously reported that sera from ovarian cancer patients contained abnormal forms of alpha-1-proteinase inhibitor (API) that predicted unresponsiveness to chemotherapy. These molecules were detected by extracting the sera with the fucose-specific lectin, lotus tetragonolobus, and analysing the extracts by electrophoresis. In a new study of ovarian cancer, we report that API cannot be extracted by lotus from fresh sera, unless the specimens are subjected to repeated cycles of freezing and thawing. Furthermore, increases in lotus-extractable API in the pretreated sera are also associated with a poor response to chemotherapy. This would suggest that lotus is extracting the same molecules in both studies. It seems likely that the discrepancy between the two studies is due to the length of time the specimens were stored prior to analysis. We present evidence suggesting that the API extracted by lotus is present in serum as soluble complexes, the detailed composition of which is unclear. The change in the properties of API in cancer may be very important with respect to tumour spread.