Flow cytometric investigations can be very complex when several parameters of cells are to be measured and cells must be sorted. On the other hand, many questions may be answered by a single-parameter analysis. The Cytomic modular data system can easily be adapted to such different needs of flow cytometry. It consists of four types of modules which can be used as stand-alone elements or can be combined into a larger data system: CYTOMIC 12, a one-and two-parameter analyzer; CYTOMIC 123, a module mainly provided for on-line three-parameter data uptake and processing; CYTOMIC SORT, a fully software-controlled sort module, where sort windows may be selected by a built-in light pen; and the CYTOMIC LIST list mode module, which is able to collect up to eight cell parameters unclassified on a 1.2-MB floppy disk. Each module built from Z80 microprocessor Eurocards has its own keyboard, display screen, power supply, analog-to-digital pulse height conversion equipment, and an intermodule data communication bus. Each module has a preprogrammed set of functions, which is specially tailored for flow cytometric applications. This kind of data system allows the user to start with a single module for one-and two-parameter analysis. When required, the system may successively be enlarged without losing the stand-alone feature of each module.