During administration of homologous nonglycosylated IL-3 to rhesus monkeys, reversal of hematologic effects and disappearance of side effects suggested a neutralizing anti-IL-3 antibody response. Among a total of 20 monkeys treated with IL-3, ELISA of serial serum samples revealed anti-IL-3 antibodies in ten animals. Antibody production tended to be dose dependent. Triplicate subcutaneous injections and i.v. administration provoked earlier appearance of antibodies than single s.c. injection. Prolonged continuous intravenous IL-3 administration (63 and 93 days) at a dose of 1 microgram/kg/day did not result in antibody production. Among a total of eight animals with sufficiently high titers to allow for antibody purification, seven appeared to have generated antibodies that neutralized the biologic activity of IL-3 in vitro. In six monkeys, the response to IL-3 decreased while antibody titers rose, strongly suggesting neutralization of IL-3 in vivo. It is concluded that recombinant, nonglycosylated IL-3 as used in this study may elicit a neutralizing antibody response.