The resurgence of aldolase isozymes in cancerous tissues is a well-known but poorly understood phenomenon. This resurgence poses the problem of whether or not adult and fetal adldolase isozymes are produced by the same cells. For clarification of this question, the immunoperoxidase technique was used to locate aldolases A, B and C in 1 type of fast-growing hepatoma, the LF hepatoma and, by comparison, in normal adult liver. Under optical microscopy, aldolases A and C were located in the cytoplasm of almost all cancerous cells. An isozyme antigenically identical with aldolase B was also present in almost all cells, but the reaction indicating the presence of this isozyme was weaker. In normal adult liver, only aldolases A and B were present in almost all hepatocytes. Under EM in LF hepatoma, the 3 isozymes were present in the cytoplasm. The 3 types of aldolase are probably present in the same cells at the same time, and they provide indirect arguments indicating that the resurgence of fetal aldolase isozymes in cancer is not the consequence of cellular selection but is due to a disturbance at the gene control level.