Monoclonal antibody WT1 (anti-CD7), conjugated to ricin A chain, was administered intrathecally to rhesus monkeys to test its suitability for use in the therapy of leukemic meningitis. The WT1-SMPT-dgRTA conjugate was cytotoxic to CEM (T-lymphoblastic leukemia) cells in vitro with an ID50 of 53 p M. Immunoperoxidase testing showed no binding of WT1 to normal human tissues other than lymph nodes. Thirteen animals received one or more intrathecal 60-μg doses of WT1-SMPT-dgRTA. All monkeys receiving repeated doses developed a cerebrospinal fluid (CSF) pleocytosis (primarily eosinophils), which was generally resolving by 3-4 weeks after therapy. Pharmacokinetic studies showed a half-life of 99 min, consistent with CSF clearance by bulk flow. Peak CSF immunotoxin concentrations exceeded the ID50 for CEM cells by more than 2 log units and a concentration exceeding the ID50 was maintained for as long as 24 h. All eight monkeys receiving repeated doses of immunotoxin developed serum antibodies against both WT1 and ricin A chain. In six of these monkeys antibodies were also present in the CSF. Both anti-WT1 and anti-(ricin A chain) antibodies were able to inhibit in vitro cytotoxicity of the immunotoxin for CEM cells; however, only anti-WT1 antibodies could block immunotoxin binding to the cell surface. No monkey developed anti-immunotoxin antibodies fewer than 7 days after the initiation of therapy, suggesting that repeated doses could be administered for up to 1 week without inhibition of clinical activity. © 1989 Springer-Verlag.