The study examined approximately 300 gang and nongang homicides that occurred between 1978 and 1981 in the jurisdiction of the sheriff's department and just over 200 homicides that occurred in the jurisdictions of 3 stations of the Los Angeles Police Department between 1979 and 1981. Case data were obtained from investigation files and included information on participants, the incidents, and police investigation activities. Particular attention was given to any changes in police definitions of gang-designated homicides over the years studied. The study also determined whether police investigative activities contributed to case designation. In the Los Angeles Police Department, there is little evidence of a change in the effects of participant, incident, and investigative characteristics on gang designation of homicides over the 3-year period. In the sheriff's department, however, characteristics that distinguished gang cases in the peak year were somewhat different than those for gang cases in the subsequent year. This was not attributable to the police investigation measures analyzed. The findings, however, are not sufficiently significant to conclude that the dramatic rise and decline in the number of gang-related homicides in Los Angeles County was merely an artifact of police practices. 10 tables and 2 references.