SummaryHost response to inoculation of foreign haemopoietic tissue was studied in C31 (C3H × 101) mice by assessing acid and alkaline phosphatase of spleen and liver homogenates. With intraperitoneal injections of LAF (C57L × A) spleen cells without prior x-irradiation, host-spleen and liver acid phosphatase and Q values were maximal at 14 days after treatment; with prior whole-body x-irradiation with 400 r, intraperitoneal injections of LAF spleen-cells gave maximal acid phosphatase values at 30 days after treatment. An intravenous injection of LAF bone-marrow suspension after 950 r whole-body irradiation doubled the acid-phosphatase activity of C31 liver tissue 14 days after treatment. In non-irradiated C31, intravenous injections of rat bone-marrow produced a 50 per cent increase in liver acid phosphatase 14 days after treatment, whereas values obtained from LAF and C31 bone-marrow injections did not differ significantly from controls. Generally, there was an increase in acid-phosphatase activity assoc...