The mechanical stability of oxyhaemoglobins A, Agenogi, C, E, F, J Toronto, Little Rock, Riverdale-Bronx, San Diego, Stanleyville-2, and Tak was found to be similar, that of A2, Duarte and D Punjab was slightly subnormal, and of Bucuresti, H, Köln, M Saskatoon, S, Shepherds Bush and Zürich was markedly decreased. Cyanide increased the stability of the latter haemoglobins.