The polymerisation of deoxyhaemoglobin S at 37 °C has been characterised by the supernatant concentration remaining after ultracentrifugation of the gel. Other haemoglobins, that are often found to occur with haemoglobin S, influence the polymerisation process. The effects of such haemoglobins have been quantitated in terms of a parameter, which estimates the difference in tendency to polymerise between a particular haemoglobin and haemoglobin S. The interpretation also assesses the participation of hybrid molecules present in mixtures of haemoglobins. The results suggest that, while each of the haemoglobins can form mixed polymers with haemoglobin S, haemoglobin F shows a greater inhibition of polymer formation than haemoglobin A, and haemoglobin A2 has the maximum inhibitory effect. Hybrids show the same gelling potential as an equimolar mixture of each of these haemoglobins (A, F or A2) with haemoglobin S. Haemoglobin C shows a large inhibition of polymerisation, intermediate to haemoglobins F and A2. However, hybrids of haemoglobins C and S behave like haemoglobin S and contribute preferentially to polymer formation. This behaviour could explain the severity of SC-disease relative to haemoglobin AS trait. © 1979.