Hydrogen cyanamide (CY) induced earlier, more uniform, and a higher percentage of budbreak in poplar (Populus alba × P. gradidentata, N C 5339).c y was phytotoxic at 0.5 M or higher concentration. Percent budbreak increased and percent dieback decreased when plants were treated with a mixture of CY and Cys. No difference was found in percent budbreak or dieback between plants treated with CY alone and a mixture of CY and Ser. Mixing CY with Cys concentrations in vitro reduced the percentage of the cyano group, and mixing of Cys with CY concentrations decreased the percentage of the SH group remaining in the mixture. Mixing CY with Ser concentrations in vitro had no effect on CY level. These studies suggest that the SH group reacts with CY directly and the improvement of budbreak and toxicity caused by mixing CY and Cys may be due to the reduction of CY concentration.