We have previously demonstrated a 33% response rate in patients with primitive neurectodermal tumours after the direct injection of 131I-monoclonal antibodies (MAbs) into the cerebrospinal fluid (CSF). Dose-limiting toxicity is myelosuppression due to the passage of the radioimmunoconjugate from the CSF to the blood compartment. This occurs at doses of 2220 MBq of 131I-MAb and above, although this is not seen in all patients studied and appears to be related to the degree of prior therapy received. Rather than attempting to improve the efficacy of this approach to the treatment of disseminated disease within the CSF compartment by dose escalation and haemopoietic rescue, we have explored the possibility of repeatedly administering the radioimmunoconjugate. Eight patients were recruited to the study, two of whom received two and six of whom received three injections of 131I-MAb. After repeated administration of 131I-MAb pharmacokinetic data revealed that, with one exception, the radioimmunoconjugate cleared from the CSF compartment with similar kinetics, while its residence time in the blood decreased with each injection. This was due to the development of an anti-mouse Ig response in the blood. Clearance of 131I-MAb from the ventricular CSF appears to be independent of the presence of an anti-mouse Ig response in this compartment. The differential clearance of the radioimmunoconjugate from the ventricular CSF and from the blood results in a marked increase in the therapeutic index that can be achieved. Up to 5920 MBq of 131I-MAb was administered as the third injection of radioimmunoconjugate and combined doses of up to 12,500 MBq were given without either haematological or neurological toxicity. These data illustrate that dose escalation and thus an increase in the dose rate delivered to tumour cells within the CSF is possible if ways are found to reduce the residence time of the radioimmunoconjugate in the blood compartment. Suggestions as to how this can best be achieved are reviewed in detail.