The concentration and the sources of heavy metals (Pb, Cd, Zn and Cu) in house dust samples of nine selected houses of Jalil Town, Gujranwala, Pakistan are determined and a comparison with the concentration of these metals in respective street dust samples is given. Sources, exterior as well as interior are identified. The extent of contribution of lead in house dust from exterior sources and interior sources is calculated by determining the isotopic ratios in house dust, street dust and paint used in the houses. It is noticed in the case of well ventilated houses, that most of the heavy metal contribution is from the exterior sources. However, in less ventilated houses, contribution from the interior sources is also significant.