Intact washed human platelets aggregated in response to paf-acether (paf) and did not metabolize [3H]paf at concentrations up to 10 nM. However, when platelets were lysed by exposure to pH 9.5, resulting in 37.5 +/- 2.5% (mean +/- SD, n = 3) lactic dehydrogenase (LDH) release, 20.5 +/- 5.7% of the radioactivity was detected as labeled lyso paf and 5.7 +/- 3.1% as labeled alkylacylglycerophosphocholine. When platelets were aggregated with 0.5 IU/mL thrombin or high concentrations of paf (100 nM), they released a part of their acetylhydrolase without releasing LDH. In supernatants obtained from aggregated platelets, 21 +/- 2% or 10 +/- 2% (n = 3), respectively, of the total platelet acetylhydrolase activity was detected vs. none in supernatants of resting cells. The release of acetylhydrolase was concentration- and time-dependent and paralleled the release of PF 4, a marker for alpha-granules. The acetylhydrolase affinity for paf (Km) measured in sonicates of resting and thrombin-activated platelets was 8.3 +/- 1.5 microM vs. 10.6 +/- 1.5 microM, n = 5, n.s. in a "Mann Whitney" test. The latter Km was slightly but significantly different (P < 0.05, n = 5) from that of the thrombin-released acetylhydrolase (7.9 +/- 1.5 microM) and that of the latter was itself different from plasma acetylhydrolase (5.3 +/- 0.5, P < 0.05, n = 5). Addition of plasma (acid-treated to inactivate acetylhydrolase) decreased the Km value of supernatant acetylhydrolase to 6.1 +/- 1.4 microM. All preparations of acetylhydrolase exhibited similar pH requirements and sensitivity to various inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)