There is an obvious discrepancy between the popularity of mistletoe extracts and their classification as a non-conventional treatment modality with unproven efficacy in oncology. The commercial preparations suffer from several major drawbacks: lack of precise declarations for the molecular characteristics and the concentrations of diverse extract constituents; the composition of extracts can even be influenced by the different methods of preparation, the time of harvest, and the type of host tree; lack of experimentally substantiated instructions for the dose of supposedly effective substance(s) and the schedule of applications to clinically trigger an undisputably documented antitumoral activity; lack of thorough clinical studies according to the generally accepted criteria as the measure for responsible recommendations. To provide the indispensable set of data for a rational decision, the immunomodulatory galactoside-specific lectin was biochemically characterized and its antitumoral/antimetastatic activity was documented in three murine tumor model systems, occurring within a narrow dose range. Biweekly treatment with s.c. injections of a lectin dose of 1 ng/kg caused no notable harmful side-effects in patients, who showed modulation of selected immune parameters. In a group of 23 patients with advanced cancer no at least partial remission was seen. In principle, enhancement of factors like cytokine availability or NK-cell activity is not necessarily linked to therapeutic benefit. Factors such as growth promotion of certain tumor cell lines by cytokines, occurrence of respective insensitivity in advanced stages or varying levels of target sensitivity to cell-mediated cytotoxicity with significant interindividual differences deserve attention. Each tumor class has to be considered separately for its responsiveness.(ABSTRACT TRUNCATED AT 250 WORDS)