Among other things, two or more different antibodies are caused to bind to one or more units of a chemical component in a sample. Each of the antibodies is attached to one or more beads (e.g., microbeads). The sample is situated on a surface of an image sensor. At the image sensor, light is received originating at a light source that is other than the beads. The received light includes light reflected by, refracted by, or transmitted through the beads. At least one image of the sample is processed to separately enumerate individual beads and complexes of two or more of the beads attached to the two or more antibodies that are bound to a unit of the chemical component. The results of the processing are used to identify a presence or a level of the chemical component in the sample.