Tryptic hydrolysis of only one of 11 studied Fc fragments of human myeloma immunoglobulins G1 (IgG1) provided an intact CH2 domain in a high yield (up to 40% as opposed to 2-3% for other IgG1s). The only structural difference of this domain was shown to be the substitution of Pro for Lys290. This decreased the capacity of the IgG1 Sem Fc fragment to interact with proteins of the complement system.