Monolayer vesicles containing pyrene-labelled nonanoyltriglyceride (1-2 ditetradecyl 3-pyrene nonanoyl glyceride) were used as a substrate to measure bovine milk lipoprotein lipase activity. The activation of lipoprotein lipase by synthetic fragments of apolipoprotein C II and apo C III was measured. Fragments 30-78 and 43-78 had actions similar to that of the entire apo C II. Fragments 50-78 and 55-78 were 50% active, fragment 60-78 was 10% active and fragment 66-78 was inactive. Thus the activating capacity depended on the length of the carboxyterminal fragment. Replacing tyrosine 62 in apo C II by glycine removed all lipoprotein lipase activating capacity, while making Tyr 62 less accessible for binding to lipids and enzyme decreased apo C II activating capacity. Apo C III1 inhibited both basal lipoprotein lipase activity (no apo C II) and lipoprotein lipase activated by apo C II. Apo C III, fragment A (1-40) which did not bind lipids, had no inhibitory effect, while fragment B(41-79) had the same effect as whole apo C III,. Apo AI, AII and C I also inhibited lipoprotein lipase. The fluorometric assay is easy to perform, and suitable for metabolic studies such as fatty-acid exchanges between lipoproteins, as it produces no alteration in the reaction products. It also avoids the use of a radio-labelled substrate.