We compared the accuracy and repeatability of 2 common methods of estimating percentage covers of sessile organisms: visual estimates and random-point-quadrats (RPQ). Comparisons of estimates were made using both quadrats in the rocky intertidal zone and simulated quadrats drawn on a computer, where estimates could be compared with true, digitized percent cover values. In each case, visual estimates were found to be more repeatable (less within-and among-observer variation) and more accurate (closer to the true value as determined by digitizing) than the RPQ method. RPQs using 100 points were more accurate and less variable than those using 50 points, but were still less accurate (and much slower to carry out) than visual estimates. The RPQ method often missed rare species (<2 % cover) altogether, but when it 'hit' them, values were usually overestunated. Visual estimates also tended to overestimate percent covers of species (although less than the RPQ method), especially uncommon ones. Thus although the probabilistic RPQ method is supposedly more objective and is statistically valid, visual estimates may give a more accurate representation of relahve coverage of sesslle organisms, and can reduce overall s a m p h g error because they make increased sample sizes possible. Use of small subdvisions in quadrats, pre-field observer training, and a conscious effort to avoid bias are necessary to make the visual method valid and accurate.

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Cell culture systems have become increasingly popular as a means of screening toxic agents and studying toxic mechanisms of drugs and other chemicals at the cellular and subcellular levels. These in vitro tests can be conducted rapidly in a broad range of relevant mammalian culture systems; a variety of biological and biochemical cytotoxicity endpoints can be examined. The following study utilized human keratinocytes to evaluate the relative cytotoxicities of nitrofurazone (NF) and silver sulfadiazine (SS), the active ingredients of FURACIN(R) Topical Cream and SILVADENE(R) Cream, respectively. These compounds are anti-infectives used in the treatment of burn patients. Cell ultrastructure and elemental composition were utilized as cytotoxicity endpoints.Normal Human Epidermal Keratinocytes (HK) were prepared from the EpiPackTM culture system (Clonetics Corporation, Boulder, CO). For scanning electron microscopy (SEM) and transmission electron microscopy (TEM), cells were seeded on sterile 35 mm Falcon plastic dishes; for elemental microanalysis, cells were plated on polished pyrolytic carbon discs (E. Fullam, Latham, NY) placed in the culture dishes.