Isothiosemicarbazones have been attracting attention of the researchers for a number of years, due to  their interesting physicochemical and structural characteristics, as well as of biological properties.   The aim of this work is the synthesis of salicylaldehyde (I) and 2-hydroxy-1-naphtaldehyde (II)  4-allyl-S-methylisothiosemicarbazones, determination of their structure and biological properties. For achieving these objectives 4-allylthiosemicarbazide was alkylated with iodomethane. After that, the corresponding aldehyde was added to the reaction medium. The resulting hydroiodide of 4-allylS-methylisothiosemicarbazone was neutralized with sodium carbonate. Crystals suitable for X-ray analysis were obtained by recrystallization in ethanol.   In C=N-NH-CS backbones of I and II the S atoms are in  trans and cis positions to  azomethine  N  atoms respectively. These  cores of the studied ligands are  essentially planar to within 0.009 and 0.026 Ǻ. In I and II the dihedral angles of aromatic rings with mentioned backbones are equal to 2.4 and 2.2° respectively. The solvent molecule and hydroiodide join I in 3D-hydrogen-bonded network (Fig. 1a), while the N-H…O hydrogen bonds link molecules of II into 1D-chains along [010] direction (Fig. 1b) The synthesized compounds selectively inhibit the human promyelocytic leukemia HL-60 cells growth in the range of concentration 10-5-10-7 mol/L.  This work was fulfilled with the financial support of the Institutional Project 15.817.02.24F.