Alloferon-1 (AF) and allostatin-1 (AS) cytotoxic and growth modulating activities have been compared. AF is cationic oligopeptide isolated from the hemolymph of experimentally infected blow fly Calliphora vicina. AS is AF synthetic analog that differs from the parent molecule in two amino acids substituted. It has been shown that both AF and AS have no direct cytotoxic activity in concentrations ranging from 1 x 10(-1) to 10 microg/ml, however, the peptides demonstrated significant effect on tumor cells proliferation in vitro. Both peptides displayed growth modulating activity in mass cell cultures and boosted growth inhibiting activity of doxorubicin in the course of P388D1 cells cloning, although AS potentated doxorubicin cytostatic activity to a greater extent. Similarly, AS boosted anti-clonogenic activity of cyclophosphamide applied in a subthreshold concentration. Experiments with peptide-fluorescein complex have demonstrated that AF and AS belong to the group of cell-penetrating peptides. Moreover, the experiments displayed AF ability to bind with chromosomes.