Forty-three Id-immune animals were bled on day 0, and levels of α Id were determined. BCL₁ cells (10⁶) were then injected and the animals were followed for onset of splenomegaly. The differences in average initial serum levels of animals that became dormant versus animals that did not were statistically significant as measured by both the Mann-Whitney U-test (P = .03) and by t-test (P = .02). Because of the necessity of killing dormant animals for analysis, only a subpopulation of dormant animals were followed for the onset of splenomegaly. The average day to splenomegaly for the dormant mice is statistically different by t-test from the average day to splenomegaly for the nondormant animal group (P = .00004). The initial levels of mouse anti-BCL₁ Id are also different for these two groups (P = .0008). Serum levels of total anti-BCL₁Id IgG were measured by radioimmunoassay as previously described.11