Lipoprotein(a) (Lp(a)) has close structural homology with plasminogen and, at least in vitro, may interfere with fibrinolysis. Any changes in the serum Lp(a) concentration during and following myocardial infarction (MI) and whether the serum Lp(a) level is affected by streptokinase (SK) are therefore of interest. Serum Lp(a) levels immediately before and 3 h after completion of an intravenous infusion of SK in 39 patients with acute MI were not significantly different (median 31.3 mg/dl before and 35.9 mg/dl after). Furthermore, SK added during the serum Lp(a) assay did not affect the result, except at very high concentrations of SK (1000 units/ml). Serum Lp(a) and fasting lipids were measured daily for 3 days following definite MI in 13 patients and then after 14 and 42 days. There was no significant change in serum Lp(a) following MI. In marked contrast, C-reactive protein levels in these patients increased steeply immediately following MI. Thus, there was no early 'acute-phase response' in serum Lp(a) levels after MI. However, greater variation in its concentration was observed at day 14 than at other times. Serum cholesterol, apolipoprotein B and apolipoprotein A1 concentrations decreased significantly following MI, whereas a significant transient increase in serum triglycerides occurred. Forty-two days after MI all lipid and lipoprotein values had regained their day 1 levels, except for apo A1, which remained depressed.