Intracytoplasmic interactions were examined in iron oxideexposed and -unexposed alveolar macrophages of mice by electron microscopy and ultrastructural cytochemistry. Submicronic-sized particles are taken up by microendocytosis into 800-Å tubules that sometimes open up into a large vacuole located midway between the plasmalemma and the Golgi complex and sometimes lead directly to that organelle, which is reactive for acid phosphatase while cells are only modestly engaged in lysosomal synthesis and then serves as the main locus for formation of heterolysosomes. In more activated cells, nascent lumbricoid lysosomes are budded directly off branches of the granular reticulum; in consequence, much of the newly synthesized acid phosphatase bypasses the Golgi apparatus, and heterolysosomes are formed throughout a widened expanse of cytoplasm. The lumbricoids exist either as free elements or as a labyrinthian network of lumbricoid units, and the two forms appear interconvertible. They are short lived in contrast to larger rounded lysosomes more commonly present, many of which are heterolysosomes as judged by their content of ingested particles. Lumbricoids readily fuse with exoplasmic structures; indeed, in some lumbricoid-producing cells terminal segments of the reticulum may share this propetry to a degree, since they resemble transitional elements when extended along large phagosomes. Microendosomes can be taken up into lumbriciods, but if the endosome is larger, lumbricoids will adhere to its surface and become part of the digestive vacuole. A labyrinth is formed to envelop large phagosomes or regions of cytoplasm marked for autophagy. The interior is invaded by branches and digestion ensues. In active macrophages the Golgi apparatus is enlarged. The centrioles organize an array of microtubules about themselves, and individual Golgi stacks are turned outward to give the organelle better access to phagosomes and lysosomes. In less active cells the microtubules disappear, and the stacks reform in a circle about the diplosome. Alveolar macrophages therefore make a variety of lysosomes. Depending on cellular activity, one kind or another predominates. Few of them fall cleanly into “primary” and “secondary” classes, and in their interactions with other cellular structures they are influenced by mechanisms operating on microscopic as well as molecular scales.