Bryostatin 1, isolated from a marine bryozoan, enhances the efficacy of cytotoxic agents through modulation of the protein kinase C pathway and is active in combination with vincristine for diffuse large B-cell lymphoma. Further, the apoptotic frequency of peripheral blood T lymphocytes as determined by flow cytometry may predict which patients will respond to this combination. We tested the efficacy and safety of bryostatin 1 50 microg/m(2) given over 24 hr and vincristine 1.4 mg/m(2) on days 1 and 15 every 28 days in aggressive B-cell non-Hodgkin lymphoma (NHL) relapsing after autologous stem cell transplantation. End points included tumor response, toxicity, and survival. Responses were correlated with an increase in apoptotic frequency of CD5+ cells by flow cytometry using annexin V staining. Fourteen patients were enrolled with 13 being evaluable for a response. The overall response rate was 31% with two patients achieving a complete response. The most common toxicities were Grade 3 lymphopenia (seven patients), Grade 3 to 4 neutropenia (two patients), and Grade 3 hypophosphatemia (two patients). Median progression-free and overall survivals for all patients were 5.7 and 21.4 months, respectively. One patient demonstrated an increase in T-cell apoptotic frequency, also achieving a complete response. Bryostatin 1 and vincristine have efficacy in select patients with aggressive NHL. Future investigations of agents targeting the protein kinase C pathway may benefit from early response assessment using flow cytometry to evaluate T-cell apoptosis.