We observed gelation of a 23-residue, peptide derived from the P-sheet domain of platelet factor-4 (PF424-46). The gels were primarily heterogeneous mixtures of 50-200 mum spherical aggregates in a less-dense gel matrix. Infrared and circular dichroism spectroscopies showed gelation involving the conversion of PF4(24-46) from random coil to beta-sheet. We used aggregation-induced NMR resonance broadening to show that temperature, pH, and ionic strength influenced PF4(24-46) gelation rates. Under. identical solution conditions, gel formation took days at T less than or equal to 20 degreesC but only 30 min at T greater than or equal to 50 degreesC. Gelation was most rapid at pH values near the pK(a) of the central His35 residue. Increases in solution ionic strength reduced the critical gelation concentration of PF4(24-46): Our results suggest that PF4(24-46) gels by 4 process combining aspects of both beat-set and beta-fibril gelation mechanisms.