The present study confirms the findings that in the two ocypodid crabs Ocypode saratan and O. ryderi haemocyanin oxygen affinity is increased in ‘replaced’ or dialysed blood compared to whole haemolymph and that this difference can be attributed to a plasma factor. After the replacement of the plasma, the haemocyanin of both species showed up to a 39% increase in oxygen affinity. The change in oxygen affinity was proportional to the logarithm of the amount of native plasma present. Further investigations have shown that this difference is not due to changes in plasma urate or bicarbonate concentrations. Using plasma exchange experiments it could be shown that the factor acts specifically on the haemocyanin of Ocypode spp. and not on Carcinus maenas haemocyanin. Fast protein liquid chromatography (FPLC) techniques indicate that the molecular weight of the factor is <5000 daltons and a specific peak could be isolated. This isolated peak is linearly correlated with changes in haemocyanin oxygen affinity. The results are discussed in the light of the overall modulation of haemocyanin oxygen affinity and the need for negative effectors.