Clinically normal psoriatic skin (CNPS) and psoriatic lesions (PLs) were studied for mast cell degranulation (MCD) in patients with acute eruptive guttate psoriasis vulgaris (AEGP) following penicillin-treated acute streptococcal throat infection. The clinically manifest duration of psoriasis at the time of the biopsies was 2, 5, 10, 14, or 21 days. Two types of MCD were distinguished. Type I was characteristic for those portions of the CNPS in which vascular and epidermal changes were detected, while the PLs showed both Type I and Type II MCD. In Type I MCD the extruded granules (MCGs) in the immediate vicinity of the mast cells appeared as intact bodies encased in a distinctly trilaminar membrane. Around subepidermal and subpapillary blood vessels, in stratum papillare without proximity of blood vessels, beneath the epidermal-dermal junction, in lamina lucida, and in intercellular space of strata basale and spinosum the MCGs appeared partly as intact structures and partly in more or less disintegrated form. In Type II MCD the MCGs were extruded without perigranular membranes. The data here presented showed that MCD is an early and constant feature in the evolution of AEGP.