Hybrid hemoglobins have been made in which one chain is derived from human hemoglobin and the other from carp hemoglobin. Both hybrid hemoglobins show low cooperativity in oxygen binding. Hybrid I (alpha carp: beta human) has a very small Bohr effect, whereas hybrid II (alpha human: beta carp) has a Bohr effect nearly as large as that for human hemoglobin. Both hemoglobins have P50's more closely resembling carp hemoglobin than human hemoglobin in the region of pH 7, and for both hybrids, as for carp, cooperativity virtually disappears at acid and alkaline pHs. Since both hybrids are formed from chains derived from cooperative parent hemoglobins, it is difficult to account for the low cooperativity in terms only of the T-state salt bridges and the alpha 1-beta 2 contacts involved in the R-T switch region. We suggest that the F9 Ser in the carp beta-chain as well as alpha 1-beta 1 interactions is important in controlling the allosteric transitions in these hybrids.