W e have previously shown that flow cytometric analysis of acridine orange-stained bone marrow cells is useful for the objective enumeration and characterization of plasma cells from patients with myeloma, frequently exhibiting an abnormal DNA and an elevated RNA content. In this report on 77 previously untreated patients, we have investigated the biologic and prognostic implications of these quantitative tumor cell parameters. The degree of marrow involvement by tumor, both by microscopic and cytometric analysis, correlated with the clinically derived tumor mass stage. Examination of the product of relative tumor cell RNA content and marrow tumor infiltrate (as a measure of metabolic capacity for immunoglobulin production) in relationship to the myeloma protein concentration in the serum revealed differences in the efficiency of immunoglobulin production and/or catabolism. There was an inverse relationship between the degree of marrow tumor involvement and RNA index, suggesting a more aggressive behavior of myeloma in patients with a low tumor cell RNA content. Prognostically, high tumor cell RNA content identified patients with a high likelihood of response to both initial treatment (32 patients, P = 0.004) and salvage therapy (29 patients, P = 0.01). Favorable factors for survival were low clinical tumor mass stage (P = 0.07) and low marrow tumor infiltrate as determined morphologically (P = 0.04) and cytometrically (P = 0.004). Thus, the direct examination of marrow cellular DNA and RNA content permitted assessment of tumor burden and was useful in the prediction of response and survival.