Blood fractions of medium-weight molecules (MWM) were isolated from healthy dogs and dogs exposed to burn injury stress (24 postburn hours). MWM caused a profound modulation of restriction-induced beta-adrenergic stress manifestations (lipolytic and glycogenolytic responses) in rats. Some MM fractions suppressed thymic hypoplasia, gastric ulceration and mediated bone marrow neutrophilic elimination in immobilized rats. Latency of insulin- induced coma was also mediated by MM. The MM fractions attenuating beta-adrenergic stress manifestations were capable of potentiating bone marrow neutrophilic depletion in immobilized rats. There was a relationship between the ability of MM to reduce thymic hypoplasia and potentiate insulin sensitivity. The findings are discussed from the biological point of view as to the stress-mediating MM properties.