Cellular and humoral markers of malignancy play several roles at many levels in the evaluation and staging of children with cancer. Cytogenetic analysis of constitutional cells can be used to determine the genetic risk of developing certain cancers, such as retinoblastoma and Wilms' tumor in high-risk families. Urinary metabolites of neuroblastoma have been studied not only for accurate diagnostic ability in children with ''small round cell" tumors, but as a screen for the presence of the tumor in large normal populations. Markers are valuable as prognostic factors at the time of cancer diagnosis; for example, the use of cell surface antigens and cytogenetics in leukemia phenotyping, leading to alterations in initial therapy. Once found at diagnosis, both specific and nonspecific markers can then be utilized to follow the regression and recurrence of a malignancy, such as serum ferritin in neuroblastoma or lactate dehydrogenase in non-Hodgkin's lymphoma. Presence of cell surface antigens to which monoclonal antibodies can be directed are becoming increasingly helpful in both tumor localization, such as in radioisotope scanning, and in therapeutic intervention, such as in purging autologous bone marrow of malignant cells prior to use as a rescue after massive cytoreduction. Finally, cellular markers have lead to a better understanding of the basic biology of particular neoplasms; for example, gene rearrangements in lymphoma, which will ultimately lead to better diagnostic and therapeutic ability.