The synthesis and antitumor activity screening of 4-aminothiazol-2(5H)-one derivatives were performed. The absence of possible 4-amino-imino tautomerism of thiazolidinones-2 has been confirmed based on the study of the molecule structures. The existence of the alone amino-form was confirmed. An anticancer activity screening was performed within the Developmental Therapeutics Program (National Cancer Institute/NIH, USA). Tested compounds possess low to moderate anticancer activity (average values - 60 cancer cell lines assay) with significant selective action on certain cancer cell lines (CCRF-CEM and RPMI-8226/leukemia, U251/CNS cancer, RFX 393/renal cancer, OVCAR/ovarian cancer etc.). The advantage of 5-ylidene-4-R-amino derivatives in comparison with compounds with free amino group was shown. Some structure-activity findings, the comparison of target compounds with isomeric 5-ylidene-2-imino(amino)thiazol-4(5H)-ones, as well as COMPARE analysis were described. Among the tested compounds (Z)-5-(furan-2-ylmethylidene)-4-(4-chlorophenylamino)thiazol-2(5H)-one (IIIk) and (Z)-5-(4-diethylaminophenylmethylidene)-4-(4-hydroxy-5-isopropyl-2-methylphenylamino)thiazol-2(5H)-one (IIIp) possessed the highest levels of activity.