In adulthood prosocialness goes beyond the behaviours of helping, caring and sharing, and implies a particular proneness to participate to other's emotional experiences and the awareness of the effects of own actions. In this paper we present a new instrument for the assessment of adult's prosocialness and we analyse its psychometric properties. Then, we present a conceptual model that assigns a central role to personal efficacy believes and agreeableness as determinants of prosocialness. 724 Italian adults participated to the research (358 males and 366 females) ranging in age from 30 to 55 years old. The factorial structure of the scale has been examined through principal components analysis, and the Cronbach's Alpha coefficient has been calculated to document its reliability. The conceptual model has been tested thorough the structural equation modelling technique. The results showed both the validity of the instrument and the plausibility of the conceptual model. Important gender differences have been observed. These findings confirm the importance of theoretical models able to consider various personal, components that operating in concert may facilitate or inhibit manifestations of prosocialness.