We report a laser experiment of astrophysical interest on radiative jet formation. Conically shaped targets are irradiated by intense laser light. An ablated plasma flow collides at the axis of the cone targets, then propagates at high Mach number, forming a jetlike structure. We measure time-resolved x-ray self-emission images from the jets. The diameter of the jet increases with decreasing atomic number of the irradiated target, suggesting that the collimation is due to radiative cooling. Two-dimensional simulations reproduce essential features of the experimental results.