Despite apparent implications of normative portfolio theory for portfolios that incorporate a wide variety of marketable security forms, most of the literature concerned with application or empirical testing of the theory has considered portfolios composed only of common stocks, cash, and the proverbial riskless bond. However, nonequity securities constitute a significant component of investors' total financial wealth, and broadly diversified securities portfolios are commonplace. The objectives of this paper are to examine the risk characteristics of 19 classes of long-term marketable securities, ranging from U.S. government bonds to speculative common stocks, and to explore some implications of these characteristics for diversification of actual securities portfolios. The first section presents some risk measures for these security classes which are derived from ex post holding period return data for the 18 years, 1951–1968. This section includes an appraisal of the efficacy of alternative approaches to the generation of the matrix of interrelationships among the returns of broad types of securities. The second section utilizes the ex post risk measures to explore the composition of minimum risk portfolios consisting of two types of marketable securities, and the final section considers the question of appropriate media for the efficient diversification of common stock portfolios.