(A) Splenomegaly, as measured by palpation, is proportional to the number of BCL₁ cells in the spleen. Naive BALB/c mice injected with 10⁶ BCL₁ cells were palpated and their spleen size was recorded single-blinded. The size was graded as 1 to 4 (spleen index), with ≥2 representing tumor growth (see the Materials and Methods). The animals were killed, the cells were stained, and the number of Id⁺ cells was determined by FACS analysis. (B) Time of onset of splenomegaly as determined by palpation correlates with the number of BCL₁ cells previously injected. Groups of five animals each were injected with graded numbers of BCL₁ cells as determined by Id analysis. Splenomegaly was determined by a single observer without knowledge of the treatment protocol. Bars are the SEM.