Young ducklings were inoculated intravenously (IV) twice with either chicken or sheep red blood cells (CRBC, SRBC). Breeder ducks were inoculated IV twice with an increased dose of CRBC. Sera of these actively immunized ducks and sera of day-old ducklings from the CRBC-inoculated breeder ducks were tested for hemagglutinins in direct-hemagglutination (DHA) tests. The young ducklings and the breeders responded with very low DHA titers to CRBC and SRBC. Although ducklings inoculated with CRBC developed a true secondary response to a second CRBC inoculation, ducklings inoculated with SRBC failed to develop elevated titers after a second of SRBC inoculation. Half the breeders failed to develop higher titers to a second CRBC inoculation. Sera from the majority of day-old ducklings hatched from eggs laid after one inoculation of the breeders had no or only minimal DHA activity. Sera of ducklings hatched from eggs laid after the second inoculation had somewhat higher DHA titers, and in greater proportion. Still, many of those duckling sera had no or minimal DHA titers. Results with Sephadex G-200 fractions indicated that hemagglutinins after one inoculation resided primarily in duck immunoglobulin M (DIgM). Sera of ducks inoculated twice with CRBC had hemagglutinating activity in DIgG, too. Hemagglutinating activity was demonstrated in both 2-mercaptoethanol-sensitive (DIgM) and -resistant (DIgG) antibodies. The conspicuously low titers in all DHA-positive sera agree with results of previous studies, suggesting that DIg's are deficient in immunological reactions requiring functional bivalency.