$\Lambda_b$ lifetime measurement using semileptonic decays at D\O

Using approximately 1 fb$^{-1}$ of data collected by the D\O detector in Run II, a sample of over 2000 $\Lambda_b \rightarrow \Lambda_c \mu X (\Lambda_c \rightarrow K_s p)$ decays has been reconstructed. We present preliminary results of a measurement of the $\Lambda_b$ lifetime using this sample.