Several reports have appeared on the use of combined radioimmunotherapy (RAIT) and chemotherapy. The choice of drug to use with RAIT and how to space the two treatments has not been completely addressed. Because every patient's cancer presents with a specific molecular phenotype, we hypothesized that it may be necessary to tailor therapy based on specific gene expression. We addressed how the form of expression of a single gene, the p53 tumor suppressor, would impact the choice of agents, as well as sequence and spacing of agents. p53 regulates cell cycle arrest to allow for DNA repair after therapy‐induced small DNA damage or induction of apoptosis if damage is great and has been shown to affect chemo‐ and radiosensitivity of cancer cells. We established 3 stable p53 transfectants of the SKOV‐3 p53null parental line (p53wt, p53143mut or p53273mut). p53 expression was confirmed using flow cytometry, using the DO1 pan‐p53 Ab and the PAb240 anti‐p53mut Ab. The colorimetric MTT assay was then used to measure dose‐dependent growth inhibition from single modality chemotherapy (doxorubicin, carboplatin, paclitaxel or topotecan) or radioimmunotherapy (90Y‐RS‐7 IgG anti‐EGP1). The % survival vs. log drug were plotted to obtain the IC50. We then used a matrix design in which we varied the sequence of the first and second modality of treatment and the spacing between the 2 treatments to determine the most synergistic and antagonistic combinations for the parental SKOV‐3 and each of the 3 transfectants. The IC50 for each therapeutic agent varied as a function of the form of p53 expressed. For example, of the 4 lines, the p53wt transfectant was the most resistant to topotecan and the 143mut was the most resistant to carboplatin. The 273mut was quite sensitive to both doxorubicin and paclitaxel, whereas the p53null and wt were not. For multimodal treatments, most combinations of RAIT and chemotherapy resulted in a 30–40% growth inhibition (GI) and were either additive or moderately antagonistic. The 3 best (>60% GI) and 3 worst (<25% GI) combinations were identified and were unique to the parental p53null and to the 3 transfectants. Certain combinations showed clear synergy and others were antagonistic, with the first treatment modality blocking the growth inhibitory effects of the second treatment modality. The form of p53 expressed affects chemosensitivity and radiosensitivity and will influence optimal multimodal therapy with RAIT and chemotherapy and the dose‐schedule (sequential with RAIT first or with drug first) when more than 1 agent is used. © 2003 Wiley‐Liss, Inc.