In multiple-input-multiple-output (MIMO) wireless communications, spatial modulation (SM) has recently emerged as a new transmission method. This letter explores the physical-layer security in typical SM systems. We present a secrecy rate analysis for multiple antenna destination and eavesdroppers receivers. Targeting against passive eavesdroppers in unknown locations, we study the efficacy of active security measure through joint signal and jamming transmission without the typical requirement of eavesdropper channel information. We demonstrate the secrecy rate and transmission power tradeoff in active source jamming by providing numerical results on achieved secrecy rate and the bit error rate (BER) at different receivers.