Progressive tumor growth in nonimmune versus Id-immune mice challenged with BCL₁ tumor cells. (A) The spleens from naive (○) and Id-immune (•) BALB/c mice injected with 10⁶ BCL₁ cells were palpated as described11 on the days indicated. Naive mice have an average spleen size of ∼0.5. Immunization with BCL₁ Id results in a spleen size of 1 to 1.5 before BCL₁ tumor injection. (B) BCL₁ cells (1 × 10⁶) were injected into naive BALB/c mice (○) or into Id-immune BALB/c mice (•). At the times indicated, mice were killed and the number of BCL₁ tumor cells was determined by staining with RtABCL₁Id for naive animals or RtAMλ for Id-immune animals (endogenous mouse anti-BCL₁ Id [MA BCL₁ Id] blocks staining with RtABCL₁-Id but not the RtAMλ Ab) and analyzed by FACS.