Background: Tisagenlecleucel, a chimeric antigen receptor (CAR) T-cell therapy, contains a murine single chain variable fragment (mCAR19) binding domain. Humoral immunity to anti-mCAR19 had no impact on safety or efficacy in pediatric r/r B-ALL patients (pts) (Mueller ASH 2017); immunogenicity in r/r DLBCL has not been studied. Methods: Tisagenlecleucel immunogenicity was measured in r/r B-ALL (ELIANA [NCT02435849, n = 75]; ENSIGN [NCT02228096, n = 29]) and r/r DLBCL (JULIET [NCT02445248, n = 99]) ≤ 12 months after infusion. Cellular immunity was measured in PBMCs and tested for mCAR19 peptide-activated T cell responses by stimulated intracellular interferon-gamma production. Anti-mCAR19 antibodies (Ig) were measured by flow cytometry at baseline and after treatment. Treatment-induced Ig was defined as the ratio of postbaseline Ig levels to baseline. The impact of preexisting and treatment-induced Ig and T-cell activation on cellular kinetics, efficacy and safety were determined. Results: 84.6% of r/r B-ALL and 91.4% of r/r DLBCL pts had preexisting humoral immunogenicity. Treatment-induced humoral immunogenicity occurred in 34.6% of r/r B-ALL and 5% of r/r DLBCL pts. No relationship was found between tisagenlecleucel expansion (AUC_0-28d) and preexisting humoral responses in r/r B-ALL (r²= 0.002) or r/r DLBCL (r²= 0.008), or treatment-induced humoral responses in r/r B-ALL (r²= 0.006). Results for C_max were similar. Treatment induced humoral responses did not impact expansion or persistence of CARs in B-ALL, but sample size prevented correlation analysis in DLBCL as only 5% of pts had treatment-induced Ig. Preexisting humoral immunity did not appear to impact transgene persistence, duration of response, event-free survival or safety in either indication. T-cell responses were consistent over time with net responses < 1% at baseline and postinfusion for the majority of pts. T-cell responses did not appear to impact transgene expansion or persistence or pt outcomes. Conclusions: Preexisting/treatment-induced humoral and antigen-specific cellular immunity did not impact tisagenlecleucel expansion, persistence, efficacy or safety. Clinical trial information: NCT02435849, NCT02228096, NCT02445248