The effect of a single LD10 dose of ara-C given to mice at various times before or after challenge with sheep red blood cells has been investigated. The 6-day hemagglutinin titer was depressed or abolished if the drug was given simultaneously with antigen, 48 hours after antigen, or 72 hours after antigen. In the latter two instances the titer recovered to control values by 12 days. The hemagglutinin titer remained depressed as long as 75 days after challenge with sheep red blood cells when ara-C was given simultaneously. It is suggested that ara-C exerts this effect by interfering with the deoxyribonucleic acid synthesis which may accompany the differentiation of monocyte to macrophage. In the Jerne plaque assay system, ara-C was immunosuppressive whether given at -24 hours, +48 hours, or +72 hours in relation to antigen or simultaneously with antigen.

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Supported in part by Grant FRO 5511 from the National Cancer Institute, NIH, Bethesda, Maryland.

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