1. Lipoprotein lipase (LPL) was isolated from five rat tissues: white adipose, skeletal muscle, cardiac muscle, mammary gland and lung. 2. Specific activity of the preparations varied from 75 U/mg for skeletal muscle and 720 U/mg for adipose. 3. The preparations were further analysed using SDS-PAGE and a single component identified. The mol. wt of 61,000 Da of this component was consistent for all five of the tissue sources. 4. Significant differences in the values of the isoelectric points of the enzyme species were revealed. The values varied from 7.23 (SEM 0.022) for cardiac and lung to 7.51 (SEM 0.037) for mammary. 5. Two-dimensional electrophoresis, using isoelectric focusing in the first dimension and SDS-PAGE in the second revealed differences in the patterns of stained material derived from the five tissue sources.