The production of adult human haemoglobin in a yeast expression system has been shown to lead to the formation of functional oxygen-binding tetrameric proteins with the incorporation of endogenously synthesized haem. Adachi et al. [(1992) Protein Engng, 5, 807-810] identified two partially resolvable forms of the expressed haemoglobin, one of which showed higher oxygen affinity and lower cooperativity than normal. We show that in contrast to the previously expressed view that the abnormal form is due to abnormal protein folding, that it represents tetrameric haemoglobin containing incorporated sulphaem. Furthermore, the incorporation of sulphaem is shown to be a time-dependent process, with no detectable sulphaem being incorporated prior to 16 h post-induction. Numerical simulation based on our analysis of sulphaem composition gives an excellent fit to oxygen binding data previously reported for samples containing mixtures of normal haemoglobin and sulphaemoglobin.