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Five multiple sclerosis patients were treated weekly with cytosine arabinoside (araC) on an escalating dose schedule. The dose was initiated at 50mg/M2 and then increased once each week by 50mg/M2 (unless toxicity caused delay). Dosage decisions were based on whether or not the antibody-dependent cellular-cytotoxicity (ADCC) or natural killer (NK) cytotoxicity levels had been reduced to a level more than 2 standard deviations below the control range. Cytosine arabinoside treatment was discontinued in 2 of 5 subjects at doses of 500mg/M2 due to toxicity. The 3 remaining patients demonstrated sustained reductions in the percentage of FcR+ cells in their peripheral blood. The maximum percentage reductions from the baseline values ranged from 50% to 76%. Concomitant reductions in the NK activity at the same doses ranged from 65% to 83%. ADCC activity in all 3 patients, however, was relatively resistant to suppression. The nadirs for the ADCC activity were only 16% to 44% below the baseline minimums. AraC was shown to reduce the proportion of FcR+ cells and NK cytotoxic activity in preference to ADCC activity.