Spin trapping is a powerful and convenient technique for the study of free radical reactions. The breadth of applications ranges from clinical studies to high-energy physics. Over 1500 references to the technique have accumulated in Chemical Abstracts. STDBII, a spin trapping database, has been implemented on IBM PC/AT and MacIntosh personal computers. The package operates with no "add-ons." The program is powerful yet user-friendly; the command structure is similar to the familiar 1-2-3 light-bar menu; search strategy employs the method of Query-by-Example (QBE); logical combinations of any fields are accomplished by using AND, OR, NOR, and EXCEPT. Presently, STDBII (4.5) contains files for 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), alpha-phenyl-N-tert-butyl nitrone (PBN), 2-methyl-2- nitrosopropane (MNP), alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (POBN), nitrosodurene (ND)and 3,5-dibromo-nitrosobenzene sulfonate (DBNBS). Data for other less popular traps are included in a catch-all file. Presently, the database files have more than 2200 references with over 9000 parameter entries. The STDBII files contain information on: 1) spin trap used; 2) radical trapped; 3) hyperfine splittings reported; 4) solvent; 5) g- value, if reported; 6) a terse description of how the radical was produced and observed; 7) full bibliographic data. The latest version (STDBII, 4.5) also contains kinetic data on spin trapping rates as well as a new parameter, the NoH value (the ratio of aN/ aH), which has proved useful for the identification of DMPO adducts. STDBII helps researchers: 1) in identification of spin adducts from the sometimes unique hyperfine splitting parameters; 2) as a key to the spin trapping literature 3) as a vehicle to correct published errors. STDBII is now available to researchers both inside and outside NIEHS. The STDBII program has been modified so that users can add their own data to the files. The spin trap database is now accessible via the Internet (http:// hippo.niehs.nih.gov/LMB/stdb/)