Summary. Differentiation therapy using retinoic acids (RAs) or 1a25-dihydroxyvitamin D 3 (D 3 ) is an attractive alternative to chemotherapy in acute myeloid leukaemia (AML) and myelodysplastic syndromes (MDS). However, with the exception of RA therapy for acute promyelocytic leukaemia (APL), RAs and D 3 are not potent enough at doses that can be tolerated by patients. We demonstrate that clofibric acid (CA) enhances the response of HL60 cells to all-trans RA and D 3 . Our findings and those of others in the field lead us to suggest that combination therapy using all-trans RA and CA should be considered as potential therapy for AML and MDS.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. The University of Chicago Press andThe American Society of Naturalists are collaborating with JSTOR to digitize, preserve and extend access to The American Naturalist.http://www.jstor.org abstract: The efficacy of entomopathogenic nematodes for biological control is assessed using deterministic models. Typically, the examination of such models involves stability analyses to determine the long-term persistence of control. However, in agricultural systems, control is often needed within a single season. Hence, the transient dynamics of the systems were assessed under specific, shortterm control scenarios using stage-structured models. Analyses suggest that preemptive application may be the optimum strategy if nematode mortality rates are low; applying before pest invasion can result in greater control than applying afterward. In addition, repeated applications will suppress a pest, providing the application rate exceeds a threshold. However, the period between applications affects control success, so the economic injury level of the crop and the life history of the pest should be evaluated before deciding the strategy. In all scenarios, the most important parameter influencing control is the transmission rate. These findings are applicable to more traditional biological control agents (e.g., microparasites and parasitoids), and we recommend the approach adopted here when considering their practical use. It is concluded that it is essential to consider the specific crop and pest characteristics and the definition of control success before selecting the appropriate control strategy. One aspect that has received considerable attention in the last 10 yr is the incorporation of explicit stage structuring in the insect-host population (Murdoch et al. 1987;Godfray and Waage 1991;Godfray 1995a, 1995b;Murdoch and Briggs 1996). Such stage structuring, in the form of coupled delay-differential equations, enables greater biological detail to be described by a few, intuitively defined parameters while maintaining a degree of analytical tractability.However, despite the increased realism of this new generation of models, it may be that, at least in terms of evaluating biological control success, they are not being used to address the pertinent questions. In recent years, a number of authors have drawn attention to the fact that long-term stability rarely has any bearing on the practical success of a control program (Murdoch et al. 1985;Kareiva 1990;Hastings 1999), although the vast majority of models continue to evaluate control success in these terms. We contend that such models of "intermediate complexity" can be used to answer specific, practical questions concerning biological control while ...

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