Lavaged rat bronchoalveolar cells were separated into four different density fractions (I-IV) by centrifugation through a discontinuous Percoll gradient. Maximum cell size was found in the lowest density fraction (I) and minimal cell size in the highest density fraction (IV), showing an inverse correlation with cell density. These fractions contained alveolar macrophages (AM) in the proportion of 97% or more by morphologic criteria, while phagocytic AM was approximately 80% in each fraction. Although the proportion of Ia antigen-positive AM was low in each fraction, it was elevated after incubation with supernatants from concanavalin A (Con A) stimulated spleen cell cultures, with greater Ia expression in higher density fractions. Differences in these fractions were also noted in several functions, including Fc receptor activity, chemotactic migration, tumoricidal activity, and interleukin 1 (IL-1) production, all of which were greater in higher density fractions (III and IV). Con A-induced T cell proliferation was, however, suppressed by these higher density fractions, whereas only an intermediate density fraction (II) enhanced T cell responses. These results indicate morphologic and functional heterogeneity among rat AM.