IgG and IgM were isolated from 10 different species (human, monkey, horse, dog, cow, sheep, rabbit, chicken, rat and guinea pig) by a combination of starch block electrophoresis and Sephadex G-200 gel filtration. The patterns of heavy and light chains were studied in urea starch gel electrophoresis at pH 3.0 after reduction and alkylation. With the exception of the chicken 7S immunoglobulin, the electrophoretic patterns of IgG from the different species were remarkably similar. However, when IgM preparations from these species were compared, marked differences in mobility of the µ chains were seen. The immunologic cross-reactions of the IgG and IgM proteins from these same species were studied by quantitative complement fixation with three rabbit antisera specific for human γ chains and three specific for human µ chains. The nonprimate IgG cross-reacted weakly (5 to 30%) compared with nonprimate IgM preparations which cross-reacted strongly (30 to 55%). In accordance with the electrophoretic studies, the chicken 7S protein was uniquely different from the other IgG fractions and did not cross-react at all with various antihuman γ chain sera while chicken IgM showed good cross-reactivity.