Interferon-alpha treatment induces complete cytogenetic remission in 25% of Philadelphia chromosome (Ph)-positive chronic myelogenous leukemia (CML) patients. These remissions are durable unlike remissions induced with other therapies and yet residual leukemia is detectable in most of these patients. Total peripheral blood mononuclear cells (PBMCs) from CML patients in long-term remission following interferon treatment exhibited significantly higher proliferative responses (four- to 15-fold over background) than normals directed against P210 BCR-ABL in extracts of transfected monkey fibroblast cells. Surprisingly, similar enhanced levels of specific proliferative responses were observed with extracts from cells expressing Bcr and/or Abl proteins. In contrast, extracts from vector only or v-Mos-expressing cells had background level responses. Control monkey fibroblast cells lacking BCR-ABL expression failed to induce proliferation over background levels. Normal individuals had no significant responses to Bcr/Abl extracts. On the other hand, peripheral blood mononuclear cells from allogeneic bone marrow transplant CML patients had proliferative responses to cell extracts independent of Bcr-Abl. These data indicate that patients in remission due to alpha-interferon treatment have significantly higher levels of specific cellular immunoreactivity against Bcr/Abl sequences than normal controls, which could play a role in maintaining cytogenetic remission in Ph-positive CML patients.