This paper reports on experiments in real-time gestural control of voice synthesis. The ability of hand writing gestures for controlling singing intonation (chironomic singing synthesis) is studied. In a first part, the singing synthesizer and controller are described. The system is developed in an environment for multi-users music synthesis, allowing for synthetic choir singing. In a second part, performances of subjects playing with the system are analyzed. The results show that chironomic singers are able to control melody with accuracy, to perform vibrato, portamento and other types of fine-grained intonation variations, and to give convincing musical performances.