The effects of systematically increasing levels of inbreeding upon growth and postnatal maternal ability were determined in laboratory mice by utilizing the reciprocal crossfostering technique. Two experiments were conducted. The first included postnatal dams which were inbred 0, 12, 25 and 38% while corresponding levels in the litter were 0, 22, 38 and 50%, respectively. The second experiment included 0, 25, 50 and 73% inbred dams with litters inbred 0, 38, 59 and 79%, respectively. Increasing the level of inbreeding was found to linearly depress postnatal maternal performance. Young which had been suckled by the more inbred dams were smaller at 12 and 21 days of age than those which had been nursed by the more outbred dams. In the second experiment, these effects were carried over into the postweaning period. Increasing levels of inbreeding in the litter significantly depressed birth weight and weight at 12, 21, 42 and 56 days. The depressing effects of inbreeding in the litter were primarily linear for both males and females under the conditions of the more moderate levels of inbreeding included in the first experiment, but were curvilinear for females and linear for males in the second experiment. The curvilinearity in females was due to the general lack of depression until inbreeding had reached 50 to 60%, after which significant inbreeding depression developed.